...batteries interchangeable, for example I have a P4, and I want to buy a P4P, are those batteries interchangeable? The batteries have different weights and flight times on DJI website? Any help/info thanks.

Yes, you get 10% extra power in the "bigger" ones, give nearly 3 minutes extra flight time, they fit in all P4 variants.

Yes, you can use the high capacity battery with the P4 and vice versa. You'll see a few extra minutes of flight time on a P4 in comparison to the regular batteries, like Nigel mentioned.