The final season was much maligned. For me, it was more than redeemed by the "The Corp is Mother, the Corp is Father" episode where we see that the villainous Bester is, within Psi Corps to other 'teeps', a pretty nice guy.
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..