As we join the race for our live coverage, the riders face 128km before the finish in Zabrze.
They face a long day in the saddle due to the 238km stage distance.
The hot weather in Europe is also making it tough. It is close to 38C in southern Poland today.
With Peter Sagan in the leader's yellow jersey and with the world champion hoping to win the stage, his Bora-Hansgrohe teammates are leading the peloton as they chase the break of the day.
The gap has fallen to 5:45 in the last 20km of racing.
Italian rider Cesare Benedetti is doing a lot of the work to keep the break under control.
The 238km stage started in Zawiercie and the riders are covering a loop south to Zabrze.
The stage profile includes numerous short climbs in the hills of Southern Poland, which will make for a hard day out for the riders and the break and for the peloton.
This is the map of the stage.
This is the profile, showing the many short climbs the riders face today.
The stage includes two feed zones due to the length of the stage and two intermediate sprints.
The 238km stage ends with three six- kilometre laps around the stadium named after Górnik Zabrze, the 14-time Polish football champion.
119km remaining from 238km
At the halfway point in the stage, the gap is stable at 5:45.
The six riders in the break of the day are Remi Cavagna (Quick-Step Floors), Bert-Jan Lindeman (LottoNL-Jumbo), Tom Skujins (Cannondale-Drapac), Nick Dougall (Dimension Data), Jan Tratnik (CCC Sprandi) and Pawel Bernas (Poland).
This is a shot via Twitter of the break in action under clear blue skies.
Despite the heat, the Polish fans have come out to cheer on the riders.
The peloton has now upped the pace, reducing the gap to the break to 4:20.
It seems that several teams are interested in ensuring the day ends in a sprint, with Matej Mohorić (UAE Team Emirates) also helping with the heavy lifting on the front.
The break of six riders leads by 4:00 as we enter the final 100km of the stage.
Peter Sagan has swapped his rainbow jersey for the race leader's yellow jersey after his huge effort to take second on the uphill finish.
Here he speaks to close friend and training partner Oscar Gatto at the start.
Dylan Teuns (BMC) parlayed a late attack on the steep finishing climb to Szczyrk to win stage 3 yesterday. It was his first WorldTour victory.
He celebrated his win but fell shy of moving into the race lead thanks to a tenacious climb by Peter Sagan.
The climb to the finish came at the end of a lumpy trek through the hills of southern Poland, with over 2,000 metres of vertical climbing and four classified ascents before the finish.
Thanks to the time bonus for second place, Sagan moved back into the yellow jersey with a six second lead on Teuns.
Sagan's teammate Rafal Majka was third across the line, and now sits in third place overall, 12 seconds off the lead of Sagan.
This is the moment Teuns celebrated his win.
To read out full stage report and see the photo gallery of the stage, click here.
We'll have a full report and gallery after today's stage finish, plus exclusive news and interviews from Alasdair Fotheringham in Poland.
The six breakaway riders have a lead of 3:40 now. They look set to stay away until the second intermediate sprint of the stage in 10km or so.
No doubt the sprinters' teams will then start the serious chase.
89km remaining from 238km
The pace has been steady all stage in the hills, with riders racing at a constant 40km/h average.
Everyone in Poland is today celebrating the 73rd anniversary of the Warsaw Uprising.
The Warsaw Uprising broke out on August 1, 1944, with Polish insurgents taking up arms against German forces. They held out for 63 days before the Germans crushed the revolt.
Some 200,000 Poles were killed and the Germans destroyed much of Warsaw in retaliation.
Even the official race cars remembered the uprising, blowing their horns.
The break has reached the second intermediate sprint in Jastrzeb Zdroj, with Bert-Jan Lindemann (LottoNL-Jumbo) taking it ahead of Jan Tratnik (CCC SPrandi Polkowice) and Pawel Bernas (Poland).
The peloton has not eased at the sprint, with the gap now down to 3:00.
75km remaining from 238km
With 75km to go, the gap is down to 2:30. It will be interesting to see if the peloton plays cat and mouse.
The peloton might want to let them hang out front to stop other attacks going clear.
As you may know, today is August 1 and so marks the official opening of the transfer window, meaning that teams can announce new signings for 2018.
Cyclingnews has reported many of the first announcements, with interviews with some of the biggest signings.
The Bora-Hansgrohe team was one of the first to make an announcement, confirming the signing of Pete Kennaugh from Team Sky and Daniel Oss from BMC.
Several other teams are also helping with the chase now as the break breaks up under the efforts of the long day out.
The break has reached the feedzone with Jan Tratnik (CCC Sprandi Polkowice) on his own as he grabs his musette.
However after a spell up front, he drops back to the rest of the break.
Now Lindeman is alone, out the back. He seems to have suffered enough today.
That leaves five up front, with their gap steady at 2:00.
Behind Benedetti for Bora is still leading the chase. He has help from UAE and Orica.
A spectator uses a hosepipe to spray the riders with water.
They need anything to help them stay cool. It is currently close to 37C out there.
The lined-out peloton also enjoyed the spray of water as they reach a sign which says 50km to go.
A close up of Peter Sagan shows him sweating just like everyone out on the road.
In other transfers news, the FDJ team has confirmed the signing of Dutch national champion Ramon Sinkeldam from Team Sunweb to further boost its lead out train for sprinter Arnaud Demare.
The French WorldTour team also confirmed the signing of experienced Grand Tour rider Georg Preidler, also from Team Sunweb.
In the peloton the FDJ team has also sent a rider to the front to help with the chase.
The riders will soon hit the 200km point, leaving 38km left to race.
The final 18km cover three laps of a six-kilometre circuit.
The speed is up to 48km/h with the five riders in the break trying to break the control of the peloton.
However they remain vigilant, with even Peter Sagan moving up close to the front to ensure his teammates are well placed for the chase and the finale.
Riders are taking long drinks from their bidons as they ride on in the heat.
38km remaining from 238km
The crowds are huge at a special sprint, with the riders fighting for the prime.
Jan Tratnik (CCC Sprandi) took the sprint.
The same huge crowds give the peloton a huge cheer too. It seems that everyone is out to see the race go by.
It's great to see Tom Skujins out front after his nasty crash at the Tour of California and subsequent time out due to concussion.
The 26-year-old Latvian also fractured his clavicle in the crash, but the more obvious injury was the concussion that caused the team to pull up alongside the rider and coax him out of the race after he had remounted and continued to chase. Skujins was later diagnosed with the fracture and concussion.
He has only ridden the Latvian national championships since the Tour of California.
Nic Dougall drops from the break to his team car.
UCI rules do not allow feeding in the final 20km of races and so the riders need to get fresh bidons and maybe a gel for the finale of the stage,
27km remaining from 238km
The UAE team is getting some help from Orica and Bora to reel in these escapees - there's no real sense of urgency yet as the gap hovers at 1:15
Bernas takes a long pull, then Cavagna takes over. The escapees still believe in their chances.
Of course UAE are working for their sprinter, stage 2 winner Sacha Modolo. Orica-Scott are in for Caleb Ewan, and Bora-Hansgrohe are protecting the lead of Peter Sagan.
Sky now come to the fore for Danny van Poppel.
24km remaining from 238km
The gap is just over a minute as Sagan sits behind his teammates, having to correct when a Sunweb rider nearly runs into him.
22km remaining from 238km
The peloton is really not in a hurry to bring the breakaway back. The gaps now out to 1:24.
Tratnik leads the breakaway under the banner for 21km to go - yes, they have an inflatable for just about every kilometer in Tour de Pologne. They've still got 1:15 as the peloton is keeping it on cruise control at the moment.
There are big crowds cheering on the breakaway as they head through the start/finish of the closing circuit for the first time. Dougall leads them in past the masses of round inflatables.
Astana contribute to the pacemaking in the peloton as the breakaway are getting increasingly desperate. Tratnik attacks - he doesn't think they're trying hard enough.
The shadows are getting quite long, and after five plus hours in the saddle, the legs are starting to burn as hot as the sun-drenched tarmac.
Cavagna attacks, Bernas is dropped. This lack of cooperation is not going to help the chances for the breakaway.
It's clear that Cavagna is a talented time trialist, his style is smooth and aerodynamic. He was the U23 national champion of France in the TT for two years in a row.
13km remaining from 238km
Cavagna has opened up a dozen seconds or so on the trio of chasers as he heads into the chute. Still 13km to go, however.
The peloton, still led by Astana, are closing in on the trio of chasers. Cavagna has 20 seconds on them, with the peloton at 1:01.
10km remaining from 238km
Bora-Hansgrohe opened up a bit of a gap with a surge at the front of the bunch - they've got the trio in sight, and make the catch with 10km to go.
Still Cavagna out front with 20 seconds.
The roads are pretty rough in parts, and Cavagna is riding on the edge looking for some smooth tarmac.
Cavagna actually has 40 seconds according to the race graphic, and the pace isn't super high in the bunch. The Polish National Team has come to the front to pitch in.
Cavagna is nearing the 1km to go banner, but he's got another full lap to hold off the chase.
Now Lotto Soudal come forward in the chasing bunch, and Cavagna is going through Avenue of the Inflatables with 24 seconds.
The peloton is in the same straightaway as the leader - and with a tailwind no less.
5km remaining from 238km
Still 14 seconds and 5.5km to go, it's not looking good for Cavagna as Gazprom helps Bora with the chase.
He's not even caught yet but Cavagna looks down at his fried quads and breathes a sigh. He's done. Caught with 5km to go.
4km remaining from 238km
4km to go and it's still Bora at the front, with FDJ making a surge from midpack to the wheel of Lotto Soudal's train.
Attack LottoNl-Jumbo with 3.8km to go.
It's Martijn Keizer. An ill-advised attack, he's not gotten much real estate at all. He'll be caught with 3km to go.
Boomerang breakaway, Keizer goes straight back. Astana pick the pace up as Sky move forward.
2km remaining from 238km
Sagan appears to be sitting this one out - he's off the Bora train, sitting well back.
There's so much chaos at the front as the lead-out men are desperately fighting for position.
Maybe Sagan learned from watching Kittel in the Tour and is hanging back to let them burn themselves out?
Here come Trek-Segafredo and Sunweb up the right side. 1km to go.
Final turn, 900m to go, Trek at the fore.
Sky lead with Van Poppel and Here comes Orica!
Bumping between Ewan and Sagan.
Ewan gets low and wobbly, but he pulls ahead and takes the win.
Looked like Danny Van Poppel was second over Sagan and Boy Van Poppel
Modolo was fifth, with Manzin behind.
Sagan gets to keep the yellow jersey - even adding some time on Teuns with the bonus for third.
1 Caleb Ewan (Aus) Orica-Scott 5:38:49 2 Danny Van Poppel (Ned) Team Sky 3 Peter Sagan (Svk) Bora-Hansgrohe 4 Boy van Poppel (Ned) Trek-Segafredo 5 Sacha Modolo (Ita) UAE Team Emirates 6 Lorenzo Manzin (Fra) FDJ 7 Tom Van Asbroeck (Bel) Cannondale-Drapac 8 Enrico Battaglin (Ita) Team LottoNl-Jumbo 9 Alan Banaszek (Pol) CCC Sprandi Polkowice 10 Roberto Ferrari (Ita) UAE Team Emirates
Sagan did bump Ewan pretty smartly in the sprint, but it seemed more Ewan's fault for coming off his teammate's wheel to the left rather than Sagan moving right.
Sagan leads his teammate Majka by 16 seconds. We suspect that Teuns is missing from the provisional results...
General classification after stage 4
1 Peter Sagan (Svk) Bora-Hansgrohe 15:41:47 2 Rafal Majka (Pol) Bora-Hansgrohe 0:00:16 3 Wilco Kelderman (Ned) Team Sunweb 0:00:23 4 Wout Poels (Ned) Team Sky 0:00:29 5 Tom-Jelte Slagter (Ned) Cannondale-Drapac 0:00:29 6 Domenico Pozzovivo (Ita) AG2R La Mondiale 0:00:31 7 Nathan Haas (Aus) Dimension Data 0:00:34 8 Vincenzo Nibali (Ita) Bahrain-Merida 0:00:37 9 Tomasz Marczynski (Pol) Lotto Soudal 0:01:01 10 Diego Rosa (Ita) Team Sky 0:01:13
Indeed he was, here are the actual top 10 GC
General classification after stage 4
1 Peter Sagan (Svk) Bora-Hansgrohe 15:41:47 2 Dylan Teuns (Bel) BMC Racing Team 0:00:10 3 Rafal Majka (Pol) Bora-Hansgrohe 0:00:16 4 Wilco Kelderman (Ned) Team Sunweb 0:00:20 5 Tom-Jelte Slagter (Ned) Cannondale-Drapac 0:00:25 6 Domenico Pozzovivo (Ita) AG2R La Mondiale 0:00:28 7 Wout Poels (Ned) Team Sky 0:00:29 8 Adam Yates (GBr) Orica-Scott 0:00:29 9 Nathan Haas (Aus) Dimension Data 0:00:34 10 Sam Oomen (Ned) Team Sunweb 0:00:34
General classification after stage 4
1 Peter Sagan (Svk) Bora-Hansgrohe 15:41:47 2 Dylan Teuns (Bel) BMC Racing Team 0:00:10 3 Rafal Majka (Pol) Bora-Hansgrohe 0:00:16 4 Wilco Kelderman (Ned) Team Sunweb 0:00:20 5 Tom-Jelte Slagter (Ned) Cannondale-Drapac 0:00:25 6 Odd Christian Eiking (Nor) FDJ 0:00:27 7 Domenico Pozzovivo (Ita) AG2R La Mondiale 0:00:28 8 Wout Poels (Ned) Team Sky 0:00:29 9 Adam Yates (GBr) Orica-Scott 0:00:29 10 Nathan Haas (Aus) Dimension Data 0:00:34
The provisional results of course are subject to change, but the last post should be accurate. Check back at our report page for full results, photos and a report to come!
We've also got a good stage of Tour of Utah today, with the mountain finish at Snowbasin.
The sun sets on this stage of the Tour de Pologne, and the generators on the inflatables are switched to off. They'll be back up and running tomorrow for stage 5 to Rzeszow. Join us for live again tomorrow! Thanks for reading.
