It sounds like the US Army is wanting to change the M4 to match the M4A1 barrel profile, which is heavy under the handguards.
You can see an M4 profile barrel here, "light under the handguards". M4A1 barrels are much thicker in this area (adding 5oz. of additional weight) in order to hold up better under full auto fire.
Testing between the two shows that the M4A1 barrel profile essentially doubles the amount of sustained full auto fire when compared to the M4 profile.
I find this change interesting, considering that the M4 is not full auto but three round burst and it took 18 30 round magazines of sustained full auto fire for the M4 profile barrel to burst.
Many US troops standard loadout is 7 magazines.
Obviously this is an extreme test that pushes the M4 carbine far past it's design and far past anything it might even be asked to perform.
Here is the link to the article I read on this change. It shows the two test videos where they first test the M4 profile barrel until it burst and then the test with the M4A1 barrel to it's limit.
This change seems like a done deal. The only thing up in the air to this point is whether or not they will retrofit the M4's already in service.