I can only comment on my experience with M&P 9Cs 1.0 and 2.0. Both had the 3.6" barrel. Again, criteria for accuracy is shooter dependent. I keep hearing about "combat accurate", in my eyes if a firearm with a minimum 3" barrel can't routinely group 3.0-3.5"/preferably less at 25 yards off sandbags, it fails my criteria. Some may say that is beyond "combat accurate". For me, in this ever changing world where the realm of self-defense, or defense of others, has evolved beyond what it was 10 years ago, it is not but that is MY criteria. Your criteria is your business and totally dependent on one's inherent capabilities. It's not unreasonable to say group size will at least double for most individuals when comparing shooting off hand to shooting from a supported rest/sandbags.
Both M&P 9Cs noticeably failed to meet my criteria even after extensive testing with multiple factory brands and reloads. The 2.0 9C would shoot Speer 124gr Speer Gold Dots well, but that was it. Drove me batty because I like M&Ps and someone will have to fight me to ever relinquish my M&P 45s, FS, Cs, and Shields, which by the way have had no issues meeting MY criteria and typically go above and beyond.
In comparison, both my FNS-9s (FS/C) meet my criteria. My S&W 3913 meets my criteria. Surprisingly, the two Canik 9s (Elite and Elite SC) recently acquired (price was just too good to pass up), are noticeably more accurate than any of the above and go above/beyond my criteria. Not to be outdone, both my Kimber Micro 9s flat out shoot even-though they are subcompacts. Shield 9 comes close enough as well.
Take the above for what it's worth. It is an observation based on MY experience/criteria with two M&P 9Cs. The FS or 4" may do noticeably better, can't say, haven't tried, and am not going to spend the $$ to find out.