TBH, Letters in Transit felt a lot like a backdoor pilot. Almost like a reboot of the series that has focused so heavily on the war between two worlds and healing that rift. The fifth season, assuming it focuses on the Observers, seems almost like an extra chapter because we weren't done with the characters yet, sort of like the Serenity movie to Firefly or the way some shows live on in comic form. Worlds Apart closed the book on the Alternate verse so well and so firmly that I actually think I would have been okay with it, provided the ending with DRJ doesn't suck.

It will of course, end on a cliffhanger, which will probably shoot us to the future using the Amber as our time machine. But I suspect S5 will be firmly in the not-so-distant future, as that is where the majority of the unresolved conflict seems to lie (all that's left of the season is to isolate DRJ on this side and stop him from ripping more holes in the universe/trying to merge them together).

They have it in them to give the show a great ending. I just have a feeling that it will end in the future and not be so rooted in the present, which I think will be...not the show I signed up for, if that is the case.