When is the Army switching to the M-4 machine gun for infantry?

It will be a while.

As of 2012 the US Army is going to buy around 140,000 M4A1 rifles from Colt Defense and Remington. These weapons will then be fitted with upgrade kits (supplier unknown) that will include a Gas Piston System or a new Direct Gas Impingement System , a new free floating rail system, and a free floating barrel.

While that is happening the US Army is running the Individual Carbine Competition witch is expected to conclude sometime between 2013 and 2014. If the winner of the competition shows a measurable improvement over the current M4, three companies will be contracted to build the new weapons which will be given to front-line troops. All other troops will receive the improved M4.

If there is minimal improvement, front-line troops will be given the improved M4 and all other troops will receive the current m4.