I don’t know the statistics these days as I’m long retired, but before I retired the military across basically all the branches was pushing for the M4 to replace the M16 in many situations. But in my time, the M16A2 was by far the most commonly fielded weapon by infantrymen with some usage of the M4 in certain roles. I believe the USMC is probably in a similar place now, with most of them using M16 (probably the M16A4 variant at this point) but M4 being used in some roles.
The M4 actually replace a submachine gun, so no you wouldn’t normally see any regular soldiers in the U.S. military using submachine guns. The rounds they fire would not have enough penetrative ability versus modern body armor and that’s essentially what the M4 is designed to be all about, small and maneuverable but with much more stopping power and penetration than a submachine gun.
That being said keep in mind that unconventional units across all four branches often use weapons totally different from what I just said, so I wouldn’t be shocked to hear reports of Marine Special Operations Capable units utilizing submachine guns especially if they were involved in highly specialist type operations where they might be going into a tight building with lots of interior walls to grab a high value target or something.