I have them and I make them.
I build integral suppressors for rifles. Although I've made several that go on pistols, I generally just order them since I am a suppressor dealer for several different company's.
I live in Arkansas and its legal to hunt with them here. Its really catching on due to urban sprawl. Lots of people are buying them just to keep from disturbing the neighbors.
I do the Ruger .22 bull barrel and integrally suppress it. Its really quiet.
I'm also soing a lot of .300 Blackouts on both bolt actions and AR's and some integrally suppressed .50's.
The paperwork isn't as big a deal as most people think it is. Its not much different than filling out a 4473 other than getting the fingerprints or the CLEO sign off. The biggest hurdle is the wait. You cant be in a rush and get one...its just a slow process sometimes taking as long as 6 months.
As for the M-4...
Although the rifle will be "hearing safe" it wont be as quiet as what you want due to the fact that any rifle that shoots a supersonic round still has the supersonic "crack" which makes it loud. The only way to really quite a rifle down is for the bullet to be subsonic...less than the speed of sound.
That is where your real gain comes from. Thats why the .300 Blackout is now the most popular thing going. Remington is loading both subsonic and supersonic ammo for it. One just has to choose the load for the application and go with it.
Very handy and very effective. The Blackout fits on AR lowers. The only difference is that its a .30 caliber barrel. Everything else is the same.