I'm new to the who B5 rights BS, but what I don't get is why hint that season 4 is the last, only to continue on with a season 5 and a spinoff, then just screw with the spinoff? Did the network get a new boss?
Entirely new network, actually. B5 was originally on the PTEN (Prime Time Entertainment) Network, a consortium of independant TV stations. By the end of season three, the structure had begun to crumble and there were just barely enough stations signed on to finance a fourth season.
It wasn't just a hint that season four would be last, that's what JMS and Doug Netter were flat-out told by WB. Then TNT came along wanting to get the rerun rights and ended up giving us the fifth season as well as the TV movies (apparently Doug Netter's quite the salesman). As it was an already successful property, they left the producers alone and even contracted for the spin-off.
Sometime near the beginning of filming the first few episodes of 'Crusade', TNT got the results of a study that showed that the B5 property wasn't increasing their viewership. When B5 came on, the regular TNT viewers went elsewhere and the B5 fans came on. When B5 went off, the B5 viewers left and the TNT folks came in. Since advertising revenue rates are determined by numbers, TNT decided they wanted out and they proceeded to make demands that made it impossible to make the show they supposedly wanted...so they shut it down.