I have a policy of nothing but complete honesty in my reviews, Thanks to all those people who sent me some very positive feedback on the 1st review(TM MP5's).
As I am not connected to any Retailer I have no desire to "talk up" ANY of the guns or Equipment I review, what you read is what you get, and I will continue to be as damn well honest as possible as I can, thanks...
Anyway this review covers my experiences with the "M4" range from Marui, the M4, M4ris and the SR16, these AEG's are closely related and use the same Upper and lower reciever (although the SR16's reciever is black rather then grey).
The M4 and the M4ris use the same telescopic stock, and the M4ris and the SR16 have the same "R.I.S." forgrip.
The Good side of the "M4" Range :-

The M4 range as a high quality feel behind it, when you pick up one of these AEG's they weigh a fair bit more then many other models, with Battery and Hicap the SR16 comes in at around 3.5kg giving it a solid feel, all the range also looks the part with a working "bolt" that allows you to quickly get to the Hop-up unit and adjust it.

Second, the AEG's work very well in combat if you are right handed, when your magazine runs dry you can extend your right index finger and press the mag release catch very easily, the mag will drop to the ground from the mag well and then with your left hand you click in a new mag with no delay whatsoever, very handy. The M4 and it's RIS brother are highly comfortable in combat with the telescopic adustable stock, you can crawl around with the AEG cradled in you arms without it being too long.

Third, the R.I.S. system allows those of you with a large wad of cash to customise your AEG till the cows come home, you can fit many useful(and useless) accesories to the RIS, and end with a gun that looks like it should be in a cheapy "straight to video" action movie, the sights on the M4 range are also very adjustable and easy to understand for the first time user, the rifle quickly melds itself to the user and "comfortable" in pretty much all combat conditions.

Forth, the internals, these AEG's field strip very quickly and easily, remove a single reciever pin from the body and the gun neatly parts in two, remaining connected by a second reciever pin at the front of the body which also acts as the fulcrum. The top of the version2 gearbox is then fully exposed and the hop-up unit and barrel can simply slide out of the foregrip for cleaning and maintainance. Remove a couple more pins here and there and you have easy access to the V2 gearbox (Except M4).

The Evil side to the "M4" range :-

Firstly the reciever question again, Argh, it has a structural flaw, the upper part is connected to the lower via two tabs which the rear pin goes through, these tabs are made of the same ABS plactic that the main body is comprised of, and because the M4 range are front heavy, IF you say.. drop the AEG from a height of 3 feet rear end first, these plastic tabs snap, and the whole gun flops in half like its back has just broken, there is NO repair for this damage, you Buy a new Upper receiver, or you buy a whole new gun. and the answer to your question is YES, I have seen this happen on the skirmish field, be careful with your M4/RIS/SR16!!

Second, on the M4 range the place where the barrel meets the reciever is put under much strain, and as such, after a short operational life the AEG tends to develop a "barrel wobble" where the whole gun seems to "sag" slightly in the middle, on the M4 this can be fixed by the fitting of a third party re-enforced steel barrel shroud, but this "Shroud" will NOT fit on the M4ris or the SR16, both of these AEG's have a plastic re-enforcer already fitted, and the forgrip internals are too complex to refit, the problem can mostly be avoided by regular maintainance, checking all the bolts and making sure they are all "tight"

Third, pins and batteries... The Reciever pins on this range (and on the TM M16A2) have a nasty habit of working loose on the skirmish field, and get instantly and permanently lost, causing a problem when your AEG's upper and lower reciever parts suddernly in the middle of combat, again you can get third party pins that have screw thread ends and are fully secure, buts that another thing you have to "fix" outside of marui... the Battery problems only affect the M4 and the M4ris, i.e. small types, with the M4 you can buy custom "horse-shoe" shaped batteries that give you a better performance, but the M4ris is completely stuck with small types, as you have to have a "fake" laser unit fixed to the top rail, this houses the tiny "small type" battery, and to change the battery on the M4ris you have to open both this laser unit AND the foregrip, disconnect the battery, thread the batts wire through a hole drilled in the top of the gun and remove it from the laser unit.... indeed a bloody awful way to house a power source.

Lastly, a note on the M4 and M4ris, upgrading these two AEG's is slighty harder due to the Gearbox connections. in the MP5 range, and the M16 range, and of course the SR16, all the gearboxes are wired with the connector coming out of the rear of the Gearbox, but the M4 and it's RIS brother both have the wire forward facing AND sealed in. mean that much more "spripping down" is needed to disconnect the gearbox from the main body of the gun, I have found that the best way to save time and effort when upgrading these is to leave the wires and forgrip alone and strip the gearbox while it is still connected to the AEG, awkward but quicker...