I was told that the series climaxed in season 4 because when the show was originally created the main writer, J. Michael Straczynski, planned out five seasons. Then the show wasn't doing so good at the end of the first season, so they asked him to cut it back to 4 seasons. He complained, but cut back his epic plot. Then it started to get popular in seasons 3 (or somewhere thereabouts), and they asked him to bring it back to 5 seasons. He complained again, because things had already been put in motion, and the show had too much momentum for it not to climax in season 4, so he largely kept the 4-season script and just added a fifth "epilogue" season on the end.