After only one rider – Armindo Fonseca – went up the road on stage three, stage four saw four riders go clear in the day’s break.
Andreas Schillinger (Bora-Argon 18), Markel Irizar (Trek-Segadredo), Alexis Gougeard (Ag2r-La Mondiale) and Oliver Naesen (IAM Cycling) formed the quartet, with the whole stage tackled at a much faster tempo than the previous day.
Schillinger was first across the intermediate sprint, jumping clear to take the maximum 15 points, while in the peloton Etixx-QuickStep hit the front to lead Marcel Kittel out.
It was Sagan who got the jump on his green jersey rivals though, with Mark Cavendish (Dimension Data) two places further back behind Kittel.
Irizar took the sole mountain point of the day, meanwhile, with team-mate Jasper Stuyven already guaranteed another day in the polka dot jersey providing he finished the stage.
Gougeard dropped back shortly after, but with Etixx-QuickStep duo Julien Vermote and Tony Martin taking turns driving the bunch, the three remaining escapees hung off the front with their advantage at less than a minute.
The gap never looked a serious threat to the peloton, with ThomasVoeckler (Direct Energie) and Lotto-Soudal also putting in some shifts on the front.
As the gap closed to within ten seconds, Schillinger was the next to sit up and Irizar and Naesen’s day up the road ended with seven kilometres remaining.