Detail level calculations can be checked in Map Pilot but the general guideline is that the P3/X3 cameras got 2.5 cm GSD at 60 meters and the Phantom 4 Pro gets 1.6 cm at the same height.
The difference between the P3 and the P4P is this:
Taken with the P3 from 60 meters
Same bush taken with Phantom 4 Pro from 60 meters.
Keep in mind that these are snapshots from Maps Made Easy web maps (not the native resolutions). The Phantom 4 Pro was a whole zoom layer closer and it was still larger in the higher zoom layer.
The change from the Phantom 3 Pro to Phantom 4 Pro corresponds to about 2.5X more pixels on target.
Also, everything comes out a lot flatter and more repeatable. Much finer vertical details are shown with the smaller GSD.