For all practical purposes it could have been the series finale instead of the season finale.

The next season will set off on a completely new arc with new characters. Other than a couple of episodes to set up the new story, It will resemble a spin off more than a 5th and final season. I have never seen a season finale with more closure

It has been a fine science fiction series, one of the best ever for pure entertainment value. Having watched the series from .the beginning, I wonder if the writers knew from the start where it was going? You would never guess by watching the 1st season