So after much tacticooling, untacticooling, a little upgrading a whole lot of researching, my M4 is finally at a point where I look at it and say "yep, that's the stuff."
I started with an inexpensive base - a "P&J" which is TaiwanGun's brand of either CYMA or A&K I think. I used to have a G&G R8-L which was decent out the box, but the plasticky feel didn't give it the same heavy-duty robustness that I like. So the P&J was cheap option to get into a full metal construction, except for grip and stock of course.
From there, I installed a tightbore barrel, new cylinder head, better quality SP100 spring and finally a new hop unit with a medium rubber. Also had the bushings and gears changed, and the motor position tuned in so I could run a higher powered battery without having to worry too much about wearing the moving parts down as quickly - it was designed for the European 420fps out the box, but in such an inexpensive gun, a little extra reliability never hurts.
Then came the extra stuff to shove on its acres of rail space - on my first gun I loaded it to the nines with everything I could get my hands on, but this time round I wanted to make sure everything had a purpose.
So I have a vertical grip which gives me a shooting position I'm really comfortable with, some rubber rail covers on each side to soften up the sharp metal edges of the RIS if at some point I'm resting my thumb or hands on the side. Also, rubber rail covers underneath either side of the grip for when I set the gun down through a window or over a wall to protect the lower rail.
The sling is a 3 point that I've fiddled with to turn into a short 1 point which works well for me at the moment, but I can always change it back if I want.
After that, I put an EOTech clone in (which really isn't as bright as I'd like, but hey ho). First this was up at the back of the receiver rail, but I had an instance where the batteries ran down during a game. So, I swapped the front "triangle" sight for a gas block/flip up sight combo, and put a PTS MBUS on the rear, moving the EOTech forward so it rests between the two top rails, which not only leaves opens up my field of view better, but also acts almost as a clamp which get rid of the little bit of wiggle between the barrel rail and the receiver. Then, because I was doing a couple of night games, I installed a tritium A2 post on the front sight, and voila.
Only other stuff is things like a lens protector for the EOTech, MagPul assists for the mags, and lots of strips of electrical tape hidden around the place getting rid of rattles (you can just about see a little on the buffer tube which makes the stock movement tight and smooth). Also there's some more behind the delta ring, on the top of each mag, inside the EOTech rail clamp and within the handguard cap. Oh and big thick double sided tape covered with black electrical tape around the 2 battery sticks so they sight nice and tight within the stock. I also cut down the plastic housing inside the buffer tube to stop the wires getting caught if I collapsed the stock all the way.