it can cause misfires, which is what you are feeling when it wont go past 4,000 rpms, the cel light should be flashing when you do this, flashing light means stop doing what your doing.
if you can get your car in the air, ramps, or jacks, hit the cat when its cold, listen for stuff rattling around inside it.if so its shot.
you can also remove the o2 sensor in the upper rear manifold and see if moves better, yes this will cause a nice exhaust leak, but its a test only. or have the exhaust pressure tested.
