It is P4. I own the P4 driver and can confirm it is the one you need for the retina MBP.
P1 is iPhone
P4 is rMBP
P5 is the battery screws on the 2009 MBP.
P2,P3, and P6 aren't currently used on Apple Products.
According to iFixit, P2 is for the iPhone:
https://www.ifixit.com/Store/Tools/P2-Pentalobe-Screwdriver-iPhone/IF145-096
and P5 is for the rMBP:
https://www.ifixit.com/Store/Tools/P5-Pentalobe-Screwdriver-Retina-MacBook-Pro-and-Air/IF145-090
and P6 is for the 2009 MBP battery:
https://www.ifixit.com/Store/Tools/P6-Pentalobe-Screwdriver-2009-MacBook-Pro-Battery/IF145-070