If an air 4 comes out this year I'd be very surprised. I can't see them already pushing the 3S into the "old model" so fast. Not the best practice for consumers imo. Mini pro series has had a much more reasonable update pattern.
The Mini 4 Pro will be turning two this year. A leaker has reiterated DJI is lining up a replacement that will launch as the DJI Mini 5 Pro, but not for another eight months or so.