Yeah... M4S is about 1" longer than regular M4. But 363mm still work. On the other hand, 509mm for M4 is way to long, unless you put a Barrel Extension aka mock silencer. Here is US foamed sound suppressor/silencer is technically illegal to own. Airsoft or not.
Anyway, we are getting off topic. And to reply Sputnik's post. Most of us don't have trigger post issue. I personally have 8 KWA AEG's that I assigned to my airsoft team locally for long term study. Each gun ran at least 50k and done of our shell have any trigger post breakage. That said, we do receive very few reports on trigger post breaking so we are still looking into this matters.