POF-USA, LWRC, Bushmaster -- looking for other companies that make a upper receiver for the M4 with a gas piston (similar to the HK416 or not)... there's also Osprey Defense and Bushmaster that make a conversion kits.

Edit. This article has some links -- Ruger SR-556 Gas Piston/Op-Rod AR-15 Carbine/Rifle: Ruger Enters the Piston-Driven AR Fray. Sarge you should read what the author says about gas piston/op-rod vs. DI. GAS -- what do you think? (This must be old: Colt M4 Carbine’s Future Uncertain: Dark Clouds Forming.)