M4 may be a bit boring compared to some of the more exotic weaponry you could pick, but lots of people do pick them for the simple reason that it is a good choice. Mags are cheap, lots of parts are available, loads of customising options open to you such as sight choices and rails etc, and you can fit a grenade launcher to it easily. It's suitable for CQB and open woodland stuff too. I'd say either the CYMA or the G&G (both are good), and if it were me, I'd go with the G&G, they look very realistic in terms of finish and work well too considering they're not that pricey. I actually use a D Boys M4 SIR, but there's nothing wrong with your choices and if I did not have that D Boys, I'd get a G&G.
I'd consider picking up a three-point sling for your M4. Some rifles actually come with one in the box, but if yours does not, they can be had for not much money. They are much better for getting the weapon up into your shoulder quickly than a basic two point sling and they mean you can run without having the weapon flapping about all over the place.
DPM 95 is okay for British woodland, but MTP works better. Up real close in woodland DPM works well, but past fifteen metres distance it is too dark, MTP works a lot better. MTP is also a bit more versatile for differing terrains, whereas DPM can really stick out in some terrains. Of course if you are doing indoor CQB it really makes little difference. Personally, I wear all black stuff, which is actually worse than either DPM or MTP in woodland, but I like it anyway, and so if you like DPM 95, go for it. Whichever camo you choose, keep in mind that you should always consider what is behind you, because that is really what makes it work, or not.
Any decent boots or even reasonably tough high ankle trainers will do the job, just make sure you can squat down in them comfortably. Personally, I wear British Army Wet Weather Boots, which are leather with a goretex lining and a vibram sole. They are comfortable to wear for long periods and flexible enough to let you squat easily.
Webbing is up to you, because it depends on whether you like to travel light or take everything including the kitchen sink. I see plenty of people wearing nothing more than an old Vietnam War era Chinese chest rig designed to hold some AK mags and they seem to do alright with that, since you can fit a lot of different mag types or other stuff into those AK pouches. Personally, I use a black 600D nylon load bearing tactical vest with shoulder padding which has a laced mesh back. It is polyurethane coated, so it's very tough and waterproof too. It has 4 mag pouches, a couple of grenade pockets, plus nylon velcro straps to attach a tactical belt. I hang a leg holster from the tactical belt on the right side and an old RAF Browning Hi Power belt holster on the left side. That works for me because I use a 300 round mag on my M4, so I can carry mags for the two pistols in the mag pockets, and if I instead use my GBB MP5K, there is enough pocket space to carry three 50 round mags for that, so even with a GBB main weapon I've got 200 rounds plus nearly 100 rounds for the two pistols. The only thing I would say about such a lightweight vest of the kind I have, as opposed to something more hefty, is that it provides no protection from hits to your sides and quite a lot of your back in the shoulder area, but I don't get hit in those places often enough for it to be a major concern. Whatever webbing you get, consider picking up a couple of caribiners, they are useful for attaching various other bits and bobs easily. Carry some adhesive tape in your webbing or a pocket too, it's really useful for running repairs on stuff and always comes in handy, especially to stop stuff flapping around.
I favour a pair of mesh goggles and a mesh lower face protector, because it is a bit fresher and cooler than a full one piece face mask and there is no possibility of it fogging my view up, nor does it restrict peripheral vision much. I also wear a replica PASGT helmet, and I recommend you get some kind of helmet as well, or at the very least a baseball cap or forage cap. A high FPS BB to the head really hurts; only this weekend I went to my car and got a couple of caps out of it that I had in the boot to lend to other players who had shown up without head protection, both of them had taken hits to the forehead, one of which had cut their skin, and they realised it was a mistake not to cover up that bit of yourself. I would also get a keffiyah scarf (shemagh), they provide a lot of neck and throat protection, and again this week I had someone on my team who got hit in the adam's apple, and that really hurt them pretty badly. Chances are, you will get a lot of BBs coming up at your head and neck because that's the bit you always have exposed when you come out of cover to aim.
I'd also consider knee and elbow pads. Being shot in the elbow hurts a lot and you will be doing a lot of kneeling and crawling, so protecting your knees is a good idea. The chances of being hit in your right elbow are pretty high, since this is very exposed when you poke around a corner to take a shot. Unless you are a lefty, in which case the same is true of your left arm.
Make sure you get an easily portable water bottle of some kind. It gets bloody hot running around with heavy gear. It doesn't have to be fancy, even an old plastic lucozade bottle will do if you don't want to spend money on some bit of army kit. Make sure it is small enough to carry easily and not be flapping around or falling out of a pocket. Stick a bigger bottle in your bag to refill it from.
If you are doing a lot of CQB, consider getting a training knife to throw or use up close, they are more use than you might think, since many sites ban you from firing at less than 1.5 metres range, so if someone is on the other side of a door, you can get them with your knife, which is a lot less painful than shooting them at 1.5 metres with something doing 300 fps.
I can relate to this cos it's what I should've done. Try lots out, research the ones you like, and make sure you are researching the right one. For example I bought a CYMA M4 because I'd seen like one review of it, and the one I got was different to the one in the review. Apparently it had the same internals, yet a polymer receiver. Now I know CYMAs aren't that good I wish I had bought a Combat Machine.
