As far as sights go you have your four main types:
Iron Sights - Standard issue and come on most of your weapons. Usually an aperture at the rear sight and a pin at the other end. Cheap, effective, durable and don't need batteries.
Reflex Sights - (Often known as red dots from video games) Not to go too sciencey on you but they work on reflecting a form of dot or cross-hair onto a piece of glass or plastic. An advantage of them is that you can comfortably keep both eyes open and see through your sights. Can be cheap or very expensive and run on batteries.
Holographic Sights - I know you speak of an eotech replica (it is not a real one most likely as they are very expensive) but that sight is unlikely (99% certain it is not unless it is hundreds of pounds) an actually holographic sight. The reason is holographic sights work on holograms to give a supposedly more accurate sight picture and reduce parallax (the problem of having two eyes in two different places), they are very accurate for real steal fire arms but not that necessary for airsoft as an airsoft gun does not give real accuracy comparable to a real M4. The sight you are viewing is probably a reflex sight as sights are classified by how they work and although it looks like a holographic sight from video games it is just visually the same and is scientifically no different to the standard to another reflex sight.
Scopes - These are quite simply for your long range shooting and are good due to their magnification. They will have numbers in the title and this is how they work: for example a 3-9x50E sight means it has magnification levels from 3-9 (A single number means fixed magnification). The 50 refers to the object lens size (in mm) which refers to how much light is let in. Some sights have an E at the end and some do not, the E refers to whether the sight has electric illumination of the cross-hair (which again requires batteries).
As far as my recommendation goes I would say for your average M4 your iron sights are more than sufficient, however, if you plan to go for an expensive scope/reflex sight that can break (which is basically everything except iron sights) get a speed BB shield as they will protect your investment (do not get a cheap chinese clone as they will give up after 1-2 shots in cqb).
Hope I helped and just remember to zero any sight you get in your garden before you skirmish with it!