The 25-year-old, who won the world title in 2016, 2017, and 2018, has only recently returned to competition after sustaining serious injury in a crash during the Tour de France last July.
The lengthy recovery period curtailed his cyclo-cross season, meaning he only finished five races prior to Sunday's World Championship, won by Dutch rival Mathieu van der Poel.
After battling for a medal on a tough Dubendorf circuit, Van Aert fell out of contention in the closing stages, finishing fourth, over two minutes behind the winner.
"I didn't come here to win this race," said Van Aert.
"To win it was obviously too soon and I knew that before I came here. I think I had my place here and I enjoyed competing in this World Championship."
Van Aert was selected for the Belgian national team with lower expectations given his injury and shorter-than-usual cyclo-cross season. The three-time world champion instead was prepared to offer his services and experience for teammate Toon Aerts.
Van der Poel rode away from his rivals on the first lap, leaving Great Britain's Tom Pidcock to take the silver medal ahead of Belgian duo Aerts in third and Van Aert in fourth.
"In the end it was a fourth place. If only riders who think they could win started here then there wouldn't be many riders on the starting line," Van Aert said.
"For sure, it's my goal to always be in cyclo-cross and next year I'm coming back to win races. Cyclo-cross is a hard sport and there is no place to hide."
After a successful season last term, Van Aert will also have road ambitions this season with his Jumbo-Visma team.
When asked how his preparations for the road season are going, he replied: "Well, it's going slow but good."