At 3/4 throttle it's posible to plane the combo, right ? If so, once on plane while sitting aft towards middle transom pull head out and check at which lower leg height is water flow passing at speed, check if passing through upper or lower plate and if with back or over transom splashes.If passing right under lower upper plate it's perfect, from there a less pitch prop maximization is the way to go.
If wanting better performance, probably can upgrade that 4 to a 5 HP by just changing carb along a less pitch prop, a matter to check tech specs for both models which should be same CC & weight.
My 5 HP OB had same issue, could not plane a 3.80 Mtr Sib with one up with an 8 pitch prop no matter if at 3/4 or full throttle, changed to a 7 pitch prop and made a night/day difference. Now planes at full throttle and remains on plane at 3/4 throttle while lightly loaded.
Happy Boating