The Max RPM range for that engine should be 4400 - 4800. You should get roughly a change of 200 rpm per 1" of pitch. So, if you're getting 5,300 rpm out of a 17" pitch, you would be looking at around 4,500 rpm out of a 21" pitch.
4500 is within the targetted operating range, but a little low IMHO. You would have slower hole shots and it will be quicker to recover and get back on plane if you end up chopping the throttle (such as when you need to cross someone's wake or come into a oddly large wave or wave set).