I certainly think you could have done it in four seasons, JMS did such a good job of wrapping up the show when he thought it was cancelled (I think season 4 is actually my favourite, the need to condense things really has him focus and there's no Grey 17 is Missing style faff, everything is about building up to the big final) most of season 5 is not only redundant but quite bad as well.
And the good stuff, mostly around the fall of Centauri Prime and Londo, is still very nice, but considering the show as done still leaves various loose ends (either as a sign "Life goes on" or because JMS was expecting either Crusade or more TV-or even a theatrical-film to pick up on them) if the show had ended a year earlier without all those dots joined up I don't think it would have mattered that much.
And the good stuff, mostly around the fall of Centauri Prime and Londo, is still very nice, but considering the show as done still leaves various loose ends (either as a sign "Life goes on" or because JMS was expecting either Crusade or more TV-or even a theatrical-film to pick up on them) if the show had ended a year earlier without all those dots joined up I don't think it would have mattered that much.