The 167.2 km Stage 4 from Bergues to Cassel climbed to a finish on the steep, cobbled ramps of the Mont Cassel — the hardest puncheur test of the week. With his team-mates offering little help in the chase of the day's breakaway, race leader Laurence Pithie spent long stretches of the finale in the wind, riding to control the race himself. On the final climb the peloton split apart on the steep ramps, and in the closing metres Natnael Tesfatsion (Movistar) produced the winning acceleration ahead of Stan Dewulf (Decathlon) and Kim Heiduk (Netcompany INEOS). Pithie came home seventh at +18s, doing enough to defend pink into the decider.

Tesfatsion's stage win and bonus moved him to second overall, seven seconds behind Pithie, with Rasmus Tiller third at +11s and Liam Slock and Stan Dewulf at +14s. Pithie held all three of his jerseys (pink, points and young rider). Movistar took the lead in the teams classification. Everything came down to the final flat stage to Dunkerque, where only bonus seconds could overturn the seven-second margin.