The Giro d'Italia champion was fifth in line in the peloton when he slipped out on a right-hand bend on the descent of Mont Brouilly, taking out Groupama-FDJ leader David Gaudu in the process.
Although Geoghegan Hart was the quicker to his feet and his bike, it was Guadu who successfully made his way back to the bunch with the help of a teammate, while it soon became apparent Geoghegan Hart was injured.
He had teammates around him at the time of the crash but was soon alone and freewheeling down the rest of the descent, throwing worried glances at his right arm and leg.
Geoghegan Hart pressed on for a few kilometres but it was soon announced he had abandoned the race and was heading to hospital.
At the finish, team director Gabriel Rasch explained to reporters that Geoghegan Hart had hit his head in the fall and felt dizzy afterwards.
"On the descent there his front wheel slipped in a corner, he was fourth position, so really bad luck really. I don't know if there was some gravel or why exactly he slipped," Rasch said.
"Then he landed on his face and his head, and his knee pretty bad, so he felt a bit dizzy and we thought it was the right decision to stop him and not take any risks."
Geoghegan Hart later took to social media to give an update of his own.
"Landed on my head today," he said. "Thank you Ineos Grenadiers the medical team for putting my long-term welfare first when it was clear I wasn’t 100%. Little rest, no screen-time & hopefully back soon."
It's another blow for Ineos Grenadiers, who lost their other GC option, former winner Richie Porte, to a crash on the opening day.
Dylan van Baarle was the team's best-placed GC rider after the stage 3 time trial but lost more than a minute on the final climb and tumbled out of the top 10 as the climbers and pre-race favourites came to the fore. Laurens de Plus finished outside the top-40 and himself fell out of overall contention.
The British team will have to turn their attention to stage wins in the remaining four stages, with Ben Swift, Rohan Dennis, and Andrey Amador the other riders remaining in the race.
Not our race this year. Sadly stage four concludes at #ParisNice without @taogeoghegan. The young Brit was forced to abandon following a crash in the closing kilometres.He will now head to hospital for further assessment. Medical update to follow later. pic.twitter.com/O8TaXk6rvuMarch 10, 2021
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