Ditch the M4 idea, get yourself a £270 ICS Galil ARM and breathe in the wood and metal goodness, the ambi-fire-selector and totally badass Tom-Sizemore-in-Heat-look.
Actually its not when compared to the guns I pointed out. With a £400 budget you can get a lot better.
Let me explain;
I never base my opinions on bias just on engineering knowledge. G&G internals are made from some of the worst quality materials in the airsoft world.
They are great if you have a low budget but the op says he has £400.
Yes they are easy to upgrade but I would rather recommend something you dont need to.
As for reliable I know an Ics plastic gun that has never had any maintenance (not even cleaned the barrel) in the 3yrs the person has had it and hes fired over a 75,000 rnds he reckons. It rattles 22rnds a second and shoots very well.
As a comparison my TM mp5k had 6mm plastic bushings, where as my cm16 had 8mm oilite bushings. And my g&g l85 has steel bushings.
My cm16 had some 50k rounds through it, with zero mods, before I decided to open it up to have a play, I could have just lubed it, and rebushed it and it would have been good. But I wanted to jack it.
Bushings are only part of the story though. Tms are a weird one to be honest the newer stuff is better but some of the older stuff you do wonder how it works so well. To be honest I would probably pick the krytac or ics over the Tm but being he wanted a cqb M4 had to chuck it in.G&G l85 isnt a combat masheep I actually quite like them. Would probably still buy the ICS l85 over it though if I was after one.
Not all G&Gs are bad but still stick by the fact of if I was going to spend £400 I wouldn't even consider one.
The OP states in his post he already has all the gear but he bought a jbbg special and now wants something better.I dont think I personally would ever say any amount of money is too much for a first gun. He gave a £400 budget and was given a £400 budget answer like he requested.
I totally understand your point of view and dont necessarily disagree but prefer just to give the OP the answer he asked for.
But as I always say every answer on here is just someones opinion not airsoft gospel.