The Pony D is better in every category than the U-4 except low speed turn rate.

The P-51D is marginally faster from 2000ft-12,000ft.
The F4U-4 out-accelerates the Mustang in all speed bands.
The F4U-4 outclimbs the Mustang at all altitudes under WEP, and the advantage is quite significant between 8000-20,000ft.
The F4U-4 is tougher.
The F4U-4 carries an extra pair of rockets.
The F4Us in general out-maneuver the Mustang.
The F4U-4 out-guns the Mustang (the P-51 loadout is either 4x .50cal with 400rds in one pair and 500rds in the second pair, or 6x .50cal with 500rds in one pair, and 270rds in the remaining four. The F4U-4 has 6x .50cal with 400rds in all guns. Either the Mustang has to reduce its volume of fire to match the Corsair's firing time, or temporarily match the Corsair's firepower at the cost of a drastic reduction in weight of fire once the four guns are dry).
The Mustang has better all-around visibility
The Mustang has longer range on internal fuel.
Yeah, I can see how the Mustang is superior in all categories.