Hmm ... having used several M4 and M4-P, and handled a few M6, I have totally missed that small detail.
That the M4-Ps are more "crunchy" with the wind-on is the most obvious difference between the two.
Can't someone show examples of this detail? - I am curious to know.
I find the M2/M3 and the later levers being equal in advancing the film, but maybe I am just lacking the finesse