Here are the differences between the two pads (materials in parenthesis are on the P4):
Front Leather - 1.35mm Microfiber (14mm Jenpro)
Nylon in Leg Channel - 400D (420D High abrasion)
Inner Core foams - HD foam, LD foam, PE foam (HD, LD & injection molded EPE)
If you're growing and will only be in the pads for a year or two, then you probably won't notice much of a difference. If you're playing at an advanced level, you're not growing anymore, and plan to play as much as your profile indicates (up to 5 times per week), then you'll want to go with the P4 because it will hold up better.