Going to larger diameter or otherwise getting more blade area (like more blades) is a double edged sword. More blade area will give you less slip, and from that the efficiency is better. BUTTTT... More blade area gives more skin friction on the blades and that hurts efficiency. For any operating speed for a given boat, there is an optimal prop. You can spec a prop for low speed. But change speed and now that prop is no longer optimal.
You worry about slip at 17-18kts, and worry about getting your 4200 full power. Optimal prop won't be the same at those two conditions.
If you run 17-18 and never go near full power, I would not worry about it making 4200. This is not some highly strung out turbo diesel. On those we get picky about making desired rpm.