Before you spend money: there's no Cicerone for EuroVelo 4 in France, and as far as I can find no English-language guidebook to it at all. The French section is branded La Vélomaritime, Roscoff to Dunkerque — about 1,450 km on ECF's figure, 1,500 on the French one, split into 53 official stages. Brittany is stages 1-13 (Roscoff to Saint-Malo), Normandy 14-40, so the bit you're after is 40 of the 53.
What exists on paper is all French. Chamina do two spiral-bound volumes: "Eurovélo 4 – Le Tour de Manche, de Cherbourg à Roscoff" (2019, ISBN 9782844664945, €15), which covers your Brittany end read backwards, and "La Vélomaritime Eurovélo 4 – de Cherbourg à Dunkerque" (2021, ISBN 9782844665430, €15), 750 km in 28 stages with mapping on an IGN 1:100,000 base — the cartography does the work even if the French doesn't. The only single volume for the whole Roscoff–Dunkerque run is Ouest-France's "Guide vélo – Vélomaritime" (2024 reprint, ISBN 9782737390456, €17.90, 224pp).
The free GPX is unusually good: lavelomaritime.fr has the whole route as one file, per-stage tracks on each stage page, and a reversed track if you ride east to west.
One number worth having before you commit. France Vélo Tourisme puts the route at 60% on road (907 km) against 39% on cycle path (584 km). It's signposted end to end, but a lot of that is open road rather than segregated path, and ECF publishes a separate "developed sections only" GPX, which tells you what isn't finished yet.
Worth knowing too: Cicerone did publish "Cycling La Vélodyssée" this year, and that one starts at Roscoff as well — if you ever fancy turning south instead of east.