assuming you are trimming it out after you plane then you are right. your speed is pretty "on" as far as the theoretical math equation goes. you need to spin a 21 pitch at 4800 rpm,s to hit 60+
has it always been like this, not going over 4k or is this boat new to you. does it plane well? if you can spin that 23p @4800 you should be in the high 60's