I think that idea works better in an MMO setting.
Witchers don't generally work in groups, certainly not teams of 4.
Imo multiplayer/coop in the Witcher games should be left until CDPR decide they want to pursue a Witcher MMORPG. Since to me, the idea of 4 witchers running around together completing contracts is kinda lore-breaking as witchers usually work alone. Not to mention female witchers supposedly don't exist, so how would you explain 4 suddenly existing?