There is a report and excerpt from a chat log at AICN that SCI FI has executed an out clause of their contract for the 5th season of Farscape, and cancelled the series. There has been no confirmation from SCI FI yet, but it is all over the boards. Note that Season 4 picks up again in January.
Update 9/9:SCI FI has now confirmed it. Its inability to grow its audience and the rising production costs are cited.