I won't say it's impossible, but Capcom would need to produce a crazy amount of content upfront in order for the game to work.
Street Fighter can get away with a small roster because its a 1v1 fighter, but not Mahvel. Unless MvC4 deliberately recycles character models from 3 then i don't see this game happening anytime soon.
Vanilla MvC3 got away with 36 characters at launch, with the models themselves not being as detailed as what were found in SFIV. It would also depend on which engine the hypothetical MvC4 goes for. For reference, SFIV uses Dimps' in-house engine, while the MvC3 games used a version of the MT Framework engine. So while yes, it did make things easier for Capcom since they were using an in-house engine, they may not have been able to carry over some of their experience from Dimps' engine from SFIV (& later SFxT). If Capcom sticks with Unreal Engine 4, Capcom may be able to carry over their knowledge from Street Fighter V to Marvel vs. Capcom 4. While they may not be able to directly rip the models of Street Fighter V (likely due to artistic reasons), they may be able to port over smaller stuff that can help streamline the development process. Likewise, it may make things easier in terms of integrating the Capcom Fighters Network into Marvel 4.
Again, this is all assuming that MvC4 happens anytime soon (which, while not impossible by any means, isn't exactly likely). At best, Marvel & Capcom would be in early talks for Mahvel 4, & that's just me being generous. If we are getting MvC4 anytime soon, we probably won't hear a peep about it until next year at the absolute earliest.