Get yourself a vert grip if you don't already have one. A Magpul MIAD gripset, they are ver comfortable. Some back up iron sights (flip up for the win), if you don't have them.
Maybe some red dot sight, or a bushnell holosight (if you are willing to spend the money).
The possibilities for an M4 are endless. It all depends on what you want. Mock silencer maybe?
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