4 Blades will always produce more stern lift than a 3B, so that's why you could stay on plane longer with the 4B. I think they generally have less slippage so they're more efficient at cruise, although the drag of the extra blade may cost you a few mph on top end.
Sounds like you just need to get your motor height corrected and then find the right combo of pitch/diameter 4B. You might try Ken at Prop Gods or try the PowerTech web site, where they have a data sheet you fill out with boat weight and the performance numbers you're getting with current prop. They will then advise what prop would best suit your application. If being able to hang on plane at low speed is a priority (which I think is important for a 20 that you run offshore), be sure to mention that.
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