my friends crx motor blew or something so he swapped out the motor and put and motor from another si motor. now the motor wont rev over 3500rpm it runs ok and everything but just that and i have checked everything and nothing the computer shoots out a code 4 so if anyone knows please any info would help!!! thanks!

 

i forgot to mentoin its a 90 crx si all original nothing added i think he put in a d15 though dont know if that makes a difference

 

Code 4-
CKP Sensor (Crankshaft Position Sensor)
defective circuit or unplugged / defective sensor

But that may not be why it won't go over 3k.
I had the same problem. It turned out to be just a loose pin in the distributor connector.
Make sure of all the connections.

 

get new rotor distubutor cap and wires spark plugs thats what fixed mine the cap was worne out and the rotor.

 

Throw on a spare known to be good dizzy and see if that's the problem. Code 4 usually happens when the CKP sensor dies and it's inside the dizzy, so a quick 3 bolts and you can solve the problem. Don't forget to clear the code before you get her running. Just pull the positive battery terminal for 10 seconds.