That might not be the IAC that could be a stuck TB plate. Take off the intake(with the car off) and open the throttle via the lever and see if there is a load of crap stuck to it.
I know someone who once had so much crap on their TB the throttle wouldn't open all the way(chevy Tahoe <---haha).
Also I'm not even sure if the IAC could allow enough air into the motor for it to climb to 4000 RPM...Think about it. The TB is what somewhere between 50-60MM The IAC hole is maybe 15mm. The volume size is a huge freakin difference.
