That would seem to be the most likely culprit. SciFi renewed the series when it was airing at 8:00 PM, with Stargate following at 9:00 PM. For season 4, SciFi reversed this, with Stargate serving as a lead-in. Farscape's ratings dropped. More to the point, Stargate's ratings were worse than Farscape's had been in the same time slot. (And this was in an environment where SciFi spent the entirety of a single broadcast night each week to running old Stargate episodes, effectively running a four hour long commercial for the series every week). To further kick the dog while it was down, SciFi consistently preempted Farscape for other stuff, or reran episodes in the time slot rather than new ones.
In Season 3 Farscape consistenly got a rating of about 2 or slightly higher. At that time, Stargate got a rating of 1.7 to 1.8 on average. When they were reversed, Stargate bumped up to close to a 2, and Farscape dropped to the 1.8 range. Despite being treated like a red-headed stepchild by the network, Farscape maintained ratings comparable to Stargate's 9:00 PM run, even while Stargate was enjoying massive promotion from the network.
SciFi executives can whine all they want about the ratings, but the fact remains that all of the available evidence indicates that they were responsible for the drop. They took a valuable property, treated it like crap, and then pointed to the fact that it didn't perform well while they were doing everything they could to kill as evidence it should be cancelled. Claims by SciFi apologists that the show was "hemorhaging viewers" are borderline idiotic, since the reason viewership was shrinking was directly related to their own treatment of the show. That is sort of like refusing to ever change the oil in your car and then complaining when the engine doesn't perform well.