What happend was that JMS was told that Season 4 was going to be the last season, TNT picked up Babylon 5 and order a new season, so he had to put a Season 5 together fast..
It's not really that he had to put season 5 together fast, the problem is there was so little sign of being able to get season 5 out that a lot of the big plot points that were originally going to be in 5 got moved forward to the end of 4 so when they finally got green lighted for 5 (after filming the intended last episode of the series, shows how much doubt there was) there wasn't much of the core plot left to work with. But IMHO the fall of Centauri Prime and the episode "View from the Gallery" (showing what it's like to be a little guy on B5) made the season worth while, yeah it lacked the focus and the oomph of seasons 3 and 4 but it was still a pretty good 22 episodes.