B5 was unique for its time because back then especially sci-fi was essentially still stuck in the syndicated monster of the week model where you had little to no continuity plot wise across episodes because it made the show easy to watch out of order and thus it could easily be syndicated.
Ironically another TV show that started as a mainly syndication format show during the 90’s - Stargate SG1 started to lose steam when they went too heavy on single plot seasons, they came to a point where they’ve essentially ran out of bad guys and they had to invent a new more and more powerful bad guy every season, and at the end even the cast seem to have given up on the show.
With the exception of the final episode, which was filmed along with Season 4 before (as you said) they knew they would get renewed for a 5th. Absolutely gut-wrenching episode.
Still, season 5 does have its moments, like the "We are one" declaration from G'Kar...