When I first started brewing French press, a (somewhat coffee snobbish) friend of mine told me that he brews his French press for exactly 4 minutes and 10 seconds. He insisted the extra 10 seconds are very important.
Coffee snob + French press is a humorous mix to me. I don't mind using one from time to time, but the poor filtration and rapidly dropping brew temperature are obvious drawbacks, and it is certainly not a fine coffee experience.
I just go with 4 minutes.