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)
The main difference between the two pads is that the P4 uses an injection molded, one piece core and the 18K uses traditional sheet foams that are glued/sewn together as the core. The injection molded core will maintain it's integrity better over time (won't shrink or break down as much), and that mold can be customized in one of four ways to suit your style. The 18K cannot be customized.