Was it simply a ratings issue? I mean the show ended on a cliffhanger (Troy came to visit and Donna took his place by accident, ending up on Antares Prime while Troy was stranded on Earth), so it was quite obvious that the producers weren't exactly ready to wrap things up quite yet. I would like to assume that they threw in the cliffhanger (https://www.youtube.com/watch?v=cO5rbTX436U) ending in hopes to persuade the powers that be to give it at least one more year.

cO5rbTX436U

The show ends on a cliffhanger. It didn't get renewed for a 5th season, even though the producers apparently counted on a renewal...

Don't take my word on it, but from what I gathered in the comments section of the YouTube video, the fifth season was supposed to involve Troy staying on Earth for most of the last season, as in order to get Donna back it would involve unfreezing everyone and appearing, making it where Burt Reynolds would have had to be in that season. However, they also couldn't be certain of Reynolds' committing for the entire season.