When I give my car gas it will only rev to 4000 rpms what could be my problem?

Hello, you had new catalytic converters installed and the problem started after? Sounds like I'd look at the installation, correct application of parts, use of proper before and after converter oxygen sensors, proper wiring, and then monitor all these components with a diagnostic tool. Something is wrong there, and you'll have to find out what. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops