add some more passive intakes to the cab. i can't say for sure what size you need without knowing a few other factors, but generally 16" sq is probably going to be a bottleneck, esp. when it's four 2x2 holes.
recommended reading:
https://www.gardenscure.com/420/cons...d-madness.html
(good info on CFM requirements, make sure your fan is up to the job)
there's another thread around, can't find it, but if i recall correctly the general idea was that you need at least 3-4 times the area in intake vs. exhaust.
this doesn't mean 12 to 16 in^2 for a 4" exhaust; a 4" exhaust is 12.56 inches of area... just a thought.
hope that helps!