M4A1 (M4) guide, hints, and tips:

This is the primary weapon for the Counter-Terrorist side of matches or scrims. This is a very solid and versatile weapon. This weapon if very accurate at long range, but still very lethal at close range. When using the M4 at long range, you are going to want to use 2-3 shot bursts. It is very helpful if you are able to either use cover at long range or to strafe then burst then strafe again then burst again. At medium range, you can take the burst up to a 3-5 shot burst. Yet again, you are going to want to try to use cover or use the strafe and burst technique. One important note is that you should not try to outpick an awper from medium to long range with the M4. At short range you can use 3-5 shot bursts again or you can even just spray at the other team's players. As mentioned before, the M4 is a very accurate gun at all ranges, this means that you can aim for the head and hit often. One weakness with this gun is that a headshot only does 82 damage when your opponent has a helmet. So you have to hit your opponent at least twice when they have a helmet.

Sources: http://www.counterstrikestrats.com/M4-Guide.asp