The different ranks between the Platinum levels are basically indicated by the P6, X4 and X5. X4 and X5 refers to Platinum Premiere and Platinum 5 Star, respectively. Nope, the number 6, in P6, does not refer to the number of years that the member have been a Plat.

Y1 : MR Basic
M1 : MR Silver
X1 : MR Gold