There is some confusion surrounding the last episode of season 4, due to the late announcement of the renewal of Babylon5 for a 5th season. JMS had the following to say about the final episode (which if there'd been no season 5 could have played at the end of season 4):
" Re: 422...this one is a stand-alone episode which I specifically designed in order to have the flexibility to air it either as 422 or as 522, depending on what happens. This way if year 4 is all there is, we get to where we need to get; if we get year 5, then we shoot 501 and air it in 422's spot, and air 422 in place of 522.
[[May/24]] 501 isn't written yet, won't be until we get the final word. We could certainly get it finished in time for the US airing in 422's spot, and as for getting it done in the UK, assuming a mid-July start for season 4, means you'd run episodes through late October/early November so again you're okay.
[[May/25]] The final 4 would get aired in October. If there's no season 5, then the fourth one aired is 422, "Sleeping in Light."
Thanks to the renewal of B5 for a 5th year, the final episode of season 4 is actually the first of the season 5 production run. The final episode of season 5 was actually filmed during season 4's production run.