Wellens showed his experience and know-how of the finish to the 96.2 km stage in Houffalize to deny the 20-year-old Hirschi a first World Tour win.
"I think I was favourite for today, which isn't easy to handle," Wellens told Eurosport. "It was a strange race today because nobody really believed in victory.
De Plus takes BinckBank GC as Naesen wins final stage
Tim Wellens snatches Binckbank Stage 4 victory and takes leader's jersey
"There was no real team to take control of the race. We were a little group in the end and we arrived at the line as a three.
Hirschi (Sunweb) led from a long way out - but with fellow Belgian rider Bjorg Lambrecht's funeral a few days ago, it was an emotional Wellens who took his place at the top of the podium.
Laurens De Plus (Jumbo-Visma) was part of the front group at the end, but he finished a distant third.
Stage 4 winner Wellens: 'Nobody believed in victory'
Wellens had previously enjoyed three victories on this stage, and he once more showed his way around the circuit in style, holding something back for just when he absolutely needed it.
Sam Bennett made it a hat-trick of wins in the opening three stages, but he was not part of the front group at the finish as Hirschi ultimately paid the price for going a little too early.
The Swiss rider had looked very sprightly, but he was held in by Wellens on the final corner, and after Hirschi opted to make a sprint for the line, the Lotto–Soudal rider tucked into his slipstream before accelerating up the final rise and catapulting himself over the line.
Tim Wellens - Marc Hirschi
Image credit: Getty Images
Ivan Garcia (Bahrain–Merida) finished in fourth place, 18 seconds behind De Plus, with Belgium's Oliver Naesen (AG2R La Mondiale) crossing the line at the same time and Michael Valgren (Team Dimension Data) in sixth position.
Stage 5 on Friday is an opportunity for the sprinters - the 184.9km route from Riemst to Venray.