to me fixed irons with optics looks funny, but its up to you, my first ar had irons on it and i scoped it and it didnt look right. if you want optics on it i would go with a red dot, more specifically aimpoint or eotech although tijicon is good too but my pockets arent that deep
If it has a fixed carry handle upper, I'd say leave it as it is. You can mount a 4x scope to the top of it, but you have a terrible cheek weld (i.e. you're chin is up on the stock, not your cheek). They also make gooseneck style mounts that will allow a red-dot style optic to be mounted forward of the carry handle and that co-witness with irons.
Now, if you have a flat-top style with removeable carry handle, and therefore a standard 1913 picatiny rail on top, feel free to mount whatever you want. Red-dots are probably the most practical, as you M4 clone is designed to primarily be a 300m or less weapon (that obviously can and will shoot father than that). You can also put a scope on it and make paper punching from 100 yds literrally boring, and also try to reach out a bit farther.