I thought the original intent was a five year series. To me the loss of Michael O’Hare was a huge break in the show’s narrative continuity. To me the Sheridan character played by Boxlietner and the background created for him was an huge ill fitting puzzle piece for the series.
The original intent was a 5 year series, but they weren’t auto green lit for 5 years. Renewal was an every year struggle and near the beginning of season 4 WB flat out said no they weren’t going to renew. So JMS started pulling story parts forward to get them told. Then literally while they were filming the last episode of the show (which is still the last episode) TNT stepped up and bought the show renewing for the 5th season.
All characters were setup with replacements just in case, Sinclair was always going to be dropped out for the Valen plot, it just wound up being earlier when O’Hare’s psychological problems got too intense and he had to drop out. So Sheridan was always coming, though some of the early beats were supposed to go to Sinclair.
Michael O’Hare suffered from severe schizophrenia and was becoming difficult to work with.
Joe covered for him and got him help and promised he would never let anyone know why he left the show until after he died.
Strazinsky blamed it on the network demanding a known quantity like Bruce Boxleitner to rescue the show. Bruce took it as a compliment and JS rewrote the show to accomodate.
Nobody knew and they were all happy to see O’Hare for his role as Valen as he managed to get his meds sorted at the time.
That’s what I know. That’s the scoop I got, from drinking with Jerry Doyle one night, he pretty much confirmed said info.