We already know that episode 11 of season 2 was supposed to be air in season 1. That makes sense because they decided not to air it and finally changed their mind.
But I can't understand why they would air the episode that introduces season 5 in the end of season 4... it's not like the plot of season 4 and 5 have anything in common.
I'm only in the middle of season 5 so I may lack some information but here is what I understood so far :
Season 4 plot :
In the end of season 3, Peter "vanished from existence". In Season 4, he manages to reappear, but William bell and David Robert Jones (who are alive since Peter never existed... are you following ?) plan to destroy the world.
Season 5 plot :
The Observers have invaded the world. The Fringe Division is trying to destroy them.
The season 5 really is a Deux Ex Machina of the end of season 4, it is not a logical consequence
In the last minute of season 4, September suddenly appears and say "They are coming".He obviously speak about the Observers. This is the Cliffhanger to season 5.
But, again, no logical link between season 4 and 5.
So... Why was episode 19 of season 4 not aired as the first episode of season 5 ?