MilitaryTimes has listed the contenders for the M4 replacement. They are:

AR-15 Platform (Direct Gas Impingement):

* Colt ACC Monolithic
* Troy Industries M7A1 (can use either DI or piston system)
* Sabre Defense Industries M5
* S&W MP-4
* Precision Reflex PRI (Upper receiver only. Attaches to M4 lower).

AR-15 Platform (Piston):

* LWRC International M6A4 IAR
* H&K 416
* Knight’s Armament SR-15
* Superior Arms S-15
* Barrett REC7 PDW
* Patriot Ordnance P416

Not based on the AR platform:

* Bushmaster/Magpul ACR
* Sig Sauer 556 Classic
* Robinson Armament XCR
* FN SCAR

Sig 556 Classic

I am not sure if this list is complete. The Beretta ARX-160 Carbine is conspicuously missing.