Do you know the gear ratio?Usually about 1.98 or 2.00. Using 2.00 ratio,19" prop at 30 mph and 12% slip your rpm is closer to
3800 not unusual for a tach to be off.Is the motor New or just new to you?
Before a prop change don't assume everything is right just because its a new install.
Is the throttle opening all the way?Is the timing set right and advancing as it should.
Perhaps you could borrow a shop tach to check your rpm against.
If I'm right at 3800 you need to gain about 600 rpm to get to about 4400. That 3 inches of pitch.
There is a lot of guess work here so don't make any changes till we know the actual numbers.
