There's nothing wrong with the P3P. I have a P3P as well as the P4, and the resulting videos and photos are almost identical. It would take a professional to pick the better photo/video when comparing. If you have a bunch of P3 batteries it makes perfect sense to pursue a P3P. Then you'll have a backup drone for training purposes (wife, kids, friends), one that you can invite a friend along to fly with you. Even though I just got a Mavic, my 3rd craft, I refuse to sell my P3 because it works so good. However, I have the old GL300A controller (July-2015 vintage) with my P3P (W323A) that I trust way more than the new GL300C that comes with the newest P3P. I've got a GL300C from my P4, mothballed, sitting in my closet because DJI detuned the C model for some reason, not sure why. I use an Inspire controller (GL658A) with my P4 to get decent range (same range as my P3P) of about 3 miles. The GL300B works good too, I had one of those also and had no complaints. However, MaxxUAS says the plastic in the GL300B is terrible, something that's important if you ever decided to modify the controller. So it might be wise to ASK what controller comes with the P3P you intend to buy. Just avoid the GL300C.
Here are the primary differences IMO that make the P4 a little better than P3P:
1. 40+mph, versus 33mph, however you can't video past 30mph without getting the props in view.
2. 25min flight times with the new 5870mah
P4P battery
3. Clip on props. Fast to install!
4. Looks sexy, a much cleaner and reliable gimbal and camera design.
Cons to P4
1. I rarely use OA in the P4 because I can only go about 18mph in that mode. Some will do 22mph. That's just too slow in most cases.
2. The GL300C controller is weak. You'll be disappointed in the range and need an antenna mod immediately ($150), or an Inspire controller ($300 on eBay).
3. Batteries cost more.
4. Camera sits 9mm higher than P3P, which puts the props in the camera view earlier.