And will there be a 4-stagger or is that it? I'm not so sure, I can't imagine how they are going to do a four-stint now, will new actors take over the main roles? To be honest, it looks to me very much after the end of the series and I like it not so much, because it is not a real happy ending.
So far there's no information if there will be a 4th season. If there's one then at the earliest in December 2019.
But if they make one, they could get back in all sorts of ways. After all, it's about time travel and so they would not have to occupy any other actors. McLaren alone will probably not happen anymore. Unless they manage to plausibly bend time again, I doubt that. Or they skip the others and continue with the time McLaren travels back to WorldTradeCenter, which I doubt.
I do not quite understand the end of Season 3 where Marcy is sitting in the bus with David but it's actually a happy ending because Marcy will probably be happy again with David.
Oh yes, and if there will be a 4th season actually depends on the number of viewers. If the series does not play enough, they will not be shooting a new season either. Lg