Mechanic said that if I could get 4000 RPMs on this motor I was doing good I think he's wrong. Have rebuild the carburetors and check the prop.
Does anyone have any ideas?

You need to find someone else.
Your Wot rpm should be 5,500-5,800 rpm
If you are getting 4,000 wot , make sure
1. You got the right size prop
2. Fuel - spark - compression
3. Check ur throttle or cables.

Your solution are here ! Fuel - spark - compression.