Add EXTRA buttons to your PS4 Controller - YouTube
PS4 PRO Modded Controller - XMOD 4 Back Buttons -Remap, Scuf, modchip – XMOD ELECTRONICS
eXtremeRate Magnetic Metal Bullet Buttons Dpads for PS4 Controller, Aluminium Thumbstick Joystick Adustable Height, Replacement Parts for PS4 Slim Pro Controllers (31 in 1) – eXtremeRate Retail
eXtremeRate Replacement Ergonomic Back Buttons, K1 K2 K3 K4 Paddles for PS4 Controller Dawn Remap Kit (Only fits with eXtremeRate Remap Kit) – eXtremeRate Retail
PlayStation 4 DualShock finally has cost-friendly back buttons: Review | Mashable
Need more buttons for your PS4 controller? This gadget adds two on the caboose | TechCrunch