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. Wherever, whenever, whoever, etc will all be revealed years down the road when we actually start talking about it.
What Chris is saying is that thinking of the next Mass Effect game as Mass Effect 4 would imply a certain linearity, a straight evolution of the gameplay and story of the first three games……….. Story-wise, the arc of the first trilogy has also been concluded, and what we will do is tell a new story set in the Mass Effect universe. That doesn't mean that events of the first three games and the choices you made won't get recognized, but they likely won't be what this new story will focus on. In other words, because the game takes place before of after the first trilogy does not mean it necessarily is a straight prequel or sequel.