I think that that the show will continue progressing in a slooooow way like it has been happening last seasons.
1 out of 8 episodes continue with the RJ story and the others are just random non interesting cases.
I have been thinking and untill the end of the show it last season 4 finale and the hole 5 season, so I amagine that season 4 finale would make thinks a little bit more confusing instead of easier in order to have enough elements for the 5 season.
I predict that the end of the story will be simple. I can't imagine a "super tricky i cant believe it" ending like RJ being PJ or similiar. I think that it is going to be 1 person that we know and that we wouldn't believe it's him.
So how would the story progress in this chapters? I think that a lot of old characters will appear again in orther to make the decition more difficult, so Mashburn will come back, Lorelei, Kristina, Rosalind... And a lot of non story episodes too till the season 5 finale that I imagine will be hurried and dissapointing for us with many things without explanation.
That's my opinion floks :)
My choice? Mashburn, for me he is the character with more posibilities. Second choice Minelli and Third Rosalind. I think that Van Pelt won't be involve but they will make us think that she is.