I understand it to be that they weren't sure there was going to be a 5th so the forced two major plot lines into Season 4 which was popular enough they got Season 5 with the problem of most of the big arcs already wrapped up.
Yeah, it basically was 5 seasons compressed into 3 seasons. The 5th season didn't even happen on the same network as the first few, IIRC.
Yep, the first four seasons were on a little network called PTEN that had four shows. The fifth was on TNT
It's not terribly unusual for a show to get dropped by a network and picked up by another; some other heavily serialized genre shows that have changed networks include Buffy (started on the WB and moved to UPN), Stargate (started on Showtime and moved to Sci-Fi) and The Expanse (started on Sci-Fi and moved to Prime Video)