Iron Sights: The M4A1′s iron sights are decent; not wonderful or annoying either. You can do pretty good with out a scope upgrade, but there is some to be gained from the upgrade.
Red Dot Sight: This sight is a good choice for giving you a clear view while aiming down the sights. It can also help you on those long range shots. This is a very good choice for the M4. The clear view it grants the user aids at all ranges.
ACOG Scope: This scope provides increased zoom while slowing down the aim time, slightly increasing recoil, but not increasing idle sway. It’s also very good scope to use in those long range fire fights. If you want to use this it is recommended that you use the kick proficiency, as this scope increases recoil a fair amount.
Holographic Sight: This sight improves short to mid-range target acquisition. It also sits higher on the mount, reducing the obscuring effect of flare and smoke through continuous fire. This is completely up to you whether you prefer it to the Red Dot.
Hybrid Sight: This sight is a mix of the Holographic sight with a 4x sight. This sight is very good in close and long range. You can switch between the 4x sight and Holographic sight. Again, this is up to personal preference. It does obstruct much of your peripheral vision, so there are trade offs to using this sight.
Thermal: The main purpose of this sight is to expose enemy movement. Enemy’s will appear as white-hot glowing through this scope. While visibility of the surrounding area is somewhat reduced, movement is highly visible. Players using Assassin will not be highlighted. Do not use this scope.