BioWare have stated that they're looking to avoid calling the next game in the Mass Effect franchise Mass Effect 4, or anything similar, as to avoid confusion.
Speaking on the company's forums, Chris Preistly, the community manager at BioWare, said "To call the next game Mass Effect 4 or ME4 is doing it a disservice and seems to cause a lot of confusion here.
"We have already said that the Commander Shepard trilogy is over and that the next game will not feature him/her. That is the only detail you have on the game. I see people saying 'well, they'll have to pick a canon ending'. No, because the game does not have to come after. Or before. Or off to the side. Or with characters you know. Or yaddayaddayadda."
BioWare have already confirmed that development of the next game has started at BioWare Edmonton, but very little else was disclosed other than the game's engine.
Source: BioWare Forums