With M3's you get 22 in the pack.
With M4's you only get 14.
Both are great diapers, though M4's are thicker and absorb more.
Price difference is very minimal (maybe like a dollar).
Personally, I like M3's more because you get 8 more diapers, and they rarely leak on me, even if they aren't as good as M4's.
What do you guys think?