It's the stock ECU. Mine does the same thing and I'm running a MSP ECU. It isn't exactly 4000 RPM, IIRC but even if I'm in vacuum (my AFC "over pressure" is set to 1.1 psi because I'm F/I) once I hit like 4500 RPM the car will peg rich. It only does this at higher RPM while cruising so it really doesn't bother me. I can cruise in 5th at 80 and it won't do that but if I downshift into 4th to pass someone it will go rich even if I don't go into boost. You could technically tune that out (4500 RPM and above throughout the vacuum range) but for me it isn't a problem because I'm rarely cruising in 4th at 80 mph.also i don't know if it's our cars in general or the FTC, but cruising around 80mph @ 4krpms after a little bit AFRs drop to 9-10 and stay there until i let off the throttle real quick. then i'm able to continue cruising until it comes back a little while again.. so i'm gonna try to find out wtf is causing it and work that out as well