Roy Yanick, director of Bioware Montreal tweeted two mildly significant tidbits about the next Mass Effect game.

They have completed 'the first pass of the Design Document' for ME4:

He also confirmed what had been suggested before: ME4 will be running on Frostbite 3 and they will be using some core systems already developed for Dragon Age: Inquisition.

In less relevant news, Drew Karpyshyn answered some more questions about his original planned ending for the Mass Effect trilogy, which I think everyone knows by now. But he also revealed one other path the writers briefly considered but ditched fairly quickly.