I'm finally done upgrading my AEG. It was a good learning experience taking the whole rifle apart, from externals to complete gearbox dis-assembly. The WE M4 AEG was surprisingly amazing right out of the box, and I say surprisingly because for the price and for WE's first AEG it compares to some high end models. It has some interesting features, such as it's functional bolt catch. It can be fully upgraded internally and externally.
I have swapped to a SHS aluminum o-ring air nozzle, Madbull Ultimate Hopup, Madbull shark bucking with a H-nub (the prometheus bucking I originally installed wasn't sitting properly in the chamber so BB's would jam), Lonex 509mm 6.03 tightbore and a JBU modular silencer for the extra length. I rear wired the gearbox so that I could install a Daniel Defense 7" lite RIS rail, thus swapping the crane stock for one that could hold the battery. My M4 will mostly be used mid range so I sat a 2.5-10x compact illuminated scope and kept the triangle sight for quick swapping to carrying handle (more suitable for close quarters). A 300 lumen tactical light with pressure switch and magpul rail covers.
I honestly think the WE M4 is a solid AEG.