I cannot wrap my mind around the cost of trying to supply the "weapon of choice" to every soldier in the field. Not to mention the ammo for each weapon. Imagine if a squad size unit say of 6 to 8 men, went out with 4 different weapons types, and 4 different calibers.

Each soldier toten his own ammo, with very limited interchangeability, should a lengthy fire fight ensue. It would be a disaster to say the least, and resupply in the field would be very difficult if any glitch in communications of what each soldier was carrying.
Standardization was the smartest thing the military every did, and it is the only practical means of insuring everyone is up and running. If one goes into all the wars fought since WWI, you will hear stories of failures of everything from bolt action rifles to machine guns. War is hard on men and equipment, and there will be failures if the weapon either CAN'T be maintained and kept reasonably clean, or the soldier does not make the effort with time permits, and with mass production assembly, and ammo manufacture.
This thing with the M4 not being reliable, is a typical criticism from soldiers, and and there are far more soldiers whom will vouch for it BEING reliable. I quizzed my son each time he got back from his three deployments, as to his opinion of the M4 and it's reliability. He said that he maintained his weapon ANY TIME they had down time, and made sure that his squad members did like wise. He never had the problems discussed here. Very few soldiers in the field can hit a man under stress at 500 or 600 years with a rifle. It is not a realistic expectation in my opinion. That is what snipers are for.
In WWI there were men that bitched about the bolt action, and men that swore by them. In WWII, there were men that whined about the M14, and men that swore by them. The M1 carbine was pathetic, but there were men that loved'em. When the M16 was introduced the wonderful government had the ammunition plants load 5.56 ammo with ball powder that was meant for the 7.62, and it burned dirty in that caliber causing premature, and extreme fouling. Because of that, the complaints never stopped, even after the problem was corrected.
I hear folks touting the AK47 as being more reliable. No doubt they are reliable. I have owned them, and shot many a round. But a guy shootin at me at 350 or 400 yards with one will damn lucky to hit me with it given it's legendary inaccuracy, and 500 to 600 yards it won't take luck, it will take a miracle. It shoots every time because the action is sloppy. I'll take the accuracy of the M4 and a cleaning kit, over the AK anytime.
I have carried the M4, and it's fore runners in the field for over 37 years and have never had it fail if maintained. I've carried it in sandy swampy North East Texas river bottoms in the hottest of summers in rain, and in below zero weather in the Texas Panhandle with blowing sand and or snow. It shot every time it needed to. It's not a bear rifle, but then again, a man is not a bear.TMTCW