^ Hmm. It seems as if, given that situation, the 'fallback' plan for S4 ought to have been the original plan, but, anyway, I digress.
I've been wondering something lately that has less to do with what 'might've happened' and more to do with my own curiosity about B5 fans' feelings on the matter: would it have made more sense for the events of A Call to Arms and Crusade to have been covered as part of the fifth season of B5 even in periphery?
Call to Arms and Crusade came a lot from direction of TNT. JMS worked closely with TNT to design a series that fit what they were looking for and tried hard to make season 5 compatible there in.
Knowing after the fact how wishy washy TNT was, I think we can infer some of the main story pushes in season 5 was because, at the time of writing the scripts for those episodes, the idea appealed to TNT, after they aired, TNT changed their mind.
To that end --- I have no proof or knowledge that this was the intent at any given point, but it wouldn't surprise me if at the start of Season 5, JMS was planning on the next series being based on the Telepath War. Direction from TNT may have changed that significantly.