4200 RPM sounds like a plug is not firing. Also does it get on plane with him by himself? If it reached the max RPM limit before I don't believe the prop is your problem. It sounds like it's only hitting on 2 cylinders. Also I'd put a "Whale Tail" or whatever they're called on the skeg to allow you to get on plane quicker. Keep us informed. And by all means if it's under warranty.... Need I say more?