most golf car engines run 2000-2800 RPM tops. The clutch should work. That's part of my job (golf cars) and I ain't seen one yet that will hold the rod(s) in the block at 4000 RPM. Some of the EZ-go cars used a little parallel twin engine. Those were kinda sweet. extremely smooth running. Made by Robin (Fuji) and it looks VERY similar to the RTV 500 /520 engine....
if you use the stock type CVT clutch, it wont' really go any faster but it'll have all the torque you'd ever want. Jerky on the throttle too, so if I were doing it, I'd figure out how to make the pedal travel as long as possible to reduce the jerkiness. Most of the gas engines never made more than 5 hp and the flywheels were really heavy to reduce throttle response for that very reason (makes it smoother).
a big problem I see is that if you use the stock clutch, you will not be able to let the engine idle without the car creeping forward (or backward). They don't have a neutral and the clutch doesn't disengage so long as the engine is runing above about 500 RPM. IT's made that way so that you just push the throttle pedal down and the car goes, you don't have to turn the key to start and put it in gear. It's for lazy golfers as we say at work. Only one I know of that will idle in gear without much creeping is the Yamaha G1 (I had one) that had a modification done so it didn't shut the engine off when you release the gas pedal. Pretty easy.
---Nivel or Red Hawk (or whoever?) may have a gear kit for your transmission, which should drastically increase speed. You have the torque to pull a higher gear for sure, where the OEM 4 hp engine does not, not without modification to the engine or an engine swap. I've done quite a few electric carts with bigger motors, different controllers, programmers, and gear kits that will achieve 35-40mph. Pretty scary to go that fast on them since the front suspension and steering geometry is such that they SUCK to drive at anything more than about 15mph.