Yeah, the M4-2000 is still one of the quietest 5.56 cans on the market, but I'd get a Silencerco Saker if I could afford another can right now. The M4-2000 is slightly quieter, but the Saker has a more durable first-blast-baffle design and it's made out of some space-age metal that's supposed to be even more durable than inconel. Also, the fact that you can switch out the bottom part of the Saker to accept other mounting options is a huge selling point for me.
Currently you can get a mount (they call it a MAAD mount) that fits their proprietary Trifecta flash hider, one that fits any of AAC's 51T muzzle devices, one that fits YHM's Y-mount, and one that's a 1/2x28 direct-thread mount, and there are sure to be more to follow. And after mounting one on my rifle I can tell you that the 51T MAAD mount goes on an AAC 51T muzzle device easier than any AAC can and also with zero play; all AAC cans have a little bit of rotational play when installed.
Don't get me wrong; I still love the AAC SDN-6 and I think it's the best all-around rifle can on the market, especially if you're running subsonic 300 Blackout. But for a dedicated 5.56 can I really think the Saker is the one to get.