Just came across this thread, and maybe got the answer for what being asked, the difference between Primacy 4 and Primacy 4 ST are:

-Both are designed for Asia Pacific Region's extreme road and weather conditions (bumpy road, potholes and thunderstorms).
-ST means Silent Tuned, available in Asia not in Europe due to drivers' prioritize less noise in Asia for apparent reasons.

As far as I concerned, verified with a tyre shop, for tyre size below 16 inches, there is no Primacy 4 for local market.
Some primacy 4 are parallel import from overseas. For this part, standard speech comes in, no warranty given by Michelin Malaysia.