The M4 is a short barreled weapon and more suitable to close quarters combat. It also tends to get heat up and get dirtier faster. Still, it seems to me that the main problem in this case is that they were simply outnumbered. If you set yourself up in a permanent fortified position, you’re going to be a target. In such a situation, the weight and bulk of your weapon is not so important a factor. The guys defending these firebases should have some heavier-duty weapons and plenty of hand grenades, grenade launchers, claymores, small mortars, etc.
You know, I’ve heard lately that the piston kits actually make the short-barreled ARs more reliable, but that they are an unnecessary expense with the longer-barreled versions, 16” and above.
Bigger issue: I see no indication that the troops had mortars, grenade launchers, and other stuff that you would want to hold off bad guys in an isolated environment.
I would also want some scoped M-14s.
M-4s are OK for patrols, but for holding an outpost in "Indian Country", you should have a whole bunch of heavy machine guns, grenade launchers, mortars, etc. Plus you should be on the high ground.