Some m4a1 tips from m4a1 guide by Globally3D

{Range}
This is one of the most important sections of any gun. Keep that in mind. In range, you must ALWAYS use estimation as to how much bullets you think you have hit and how much more. In scrims, this is very important, because not only will you encounter firefights across many types of distances, but it can also relate to the positioning as mentioned earlier. This is where common sense plays in.

-Short Range-
Simple enough. If your crosshair is placed in a way such that their head will somehow cross over it, just burst. If you are rushing or they are in close proximity to your character, spray in an efficient and effective manner. If your bullets are missing, you'll want to burst 5-6 shots. You should also keep in mind that the average M4A1's spray pattern looks like a T. Make sure that you pull down according to the timing. First, the bullets start at the bottom of the straight line of the T. It'll move up and suddenly jerk somewhat diagonally leftwards to the left line of the T, then will move horizontally right to the right side of the T. Ballpark the estimation required to time your spray control directions and use it to your advantage, which of course, helps to fight those M-12 users you've been wanting to get revenge on.

-Mid Range-
This T pattern still applies to bursts when the recoil hasn't had enough time to cool down. Therefore in mid-range combat, burst in 2-3 or even sometimes 4 shots while properly strafing as mentioned above. Location really matters here. If you're at a higher spot, chances are, it's much more easier to get your head blown off from another M4 user, because your head and chest is quite more easy to aim at. (from my experiences, anyway)
If you're lower, you'd want to aim for the head of the enemy who is above you.

-Long Range-
Headshots and chest shots. Two terms. Do not hit the waist below, or you're simply wasting your time. Although the M4 is accurate, wasting too much time allows you to be open to flanks, snipers, etc. This is why 1-2 shot taps are recommended, while strafing in large-small steps in combination. (longright-shortleft-shortright-longleft is a good strafing
combination which allows a sniper to potentially miss his shots, or a rifle user which will have to reposition his crosshair)

The pros:
It's very accurate. No reason as to why it is not. Custom is more accurate and therefore shouldn't be mentioned in this guide. Second, because it's accurate, play relaxed but not lazy. Especially in a scrim when you know you gotta clutch that 1v4, playing too serious will cause you to overthink in almost everything you do. (recoil control, strafing, burst control, etc) Instead, play without panic. Play without fear. If you're in that clutch situation, make sure you check your back. The pros of any gun come with the player's common sense.

The cons:
It's very weak. If the enemy is standing still, try to go for the headshot. 4 shots does take very long especially if you're at shooting over a long distance or if the target is behind boxes. Not only is it very weak, but it also increases in recoil after about a 5-round burst. Even controlled, it is still very difficult to properly estimate the spray pattern changes during battle.