Been there... done that!
And here is my advice:
First, make sure the prop you have now gives you close to the desired performance. Plug the numbers from your runs into the prop slip calc. The more props/data you have to try, the better! Use the prop slip calc to ensure you are not going to over-rev the engine!!!!!
http://www.go-fast.com/Prop_Slip_Calculator.htmIf going to a 4 blade prop, remember as the pitch increases, the diameter decreases. This means you may be losing a little bite/ blade area at top-end. As you indicated, hole-shot will be improved, as well as slow speed control/stability, but expect to lose about 2-3 mph with the same pitch prop as your three blade.
Personally, I was slightly disappointed with the performance of a 4 blade on my 210SS. Instead, I stepped up from the 19P 3bl SS to a 21P hi-five. I actually only lost about 2 mph (now 53 mph at WOT/ 5400 RPM), and gained incredible hole-shot and stability. Not to mention insane control when making hard turns at high throttle/speed. I have always used these guys close to me, and find they are a very good price...
http://midwestpropeller.com/Good luck!
_________________
Walt B
"Debt-Charged"Black '07 H210SS6.2L Bravo One turning a 21P HIFive
Wakeboard Tower with Perfect Pass Stargazer version
Missouri River; Kansas City, Mo.