Ok, I will bite. How do you know you have cavitation at 4500rpm....or any rpm?
I can see a BIG problem in the pic in post #1: the impeller. See how the back of it is open? Just perfect for the water to escape....& NOT be captured & pumped through the engine. Make a donut out of thin steel or alum, OD same as impeller. Cut it in half. Slide the halves behind the imp & pop rivet/weld/braze in place. Now all the water gets captured & pumped through the engine.