The replacement showrunner for the extended Season 4 run, Erica Shelton Kodish, left the show not long ago to enter her own overall deal, with this one happening at CBS.
So perhaps all involved thought it best to close the story out without needing to find another ship captain to carry on an entire season.
After Season 3, creator Mara Brock Akil stepped down as showrunner and, along with her husband/co-executive producer Salim Akil, entered into an overall deal with Warner Bros., for whom they’re putting together The CW’s super-drama Black Lightning; both remained on in an executive producer capacity.
While it’s not entirely clear yet why BET is choosing to go with a TV movie over a full final season, it might have something to do with the changes that went on behind the scenes going into and coming out of Season 4.