well it basiclly depends on if you want a automatic or just a spring m4. If you get an automatic they can break very eaisly unless you take care of it. My suggestion is to go to www.Hobbytron.com. one of the best sites plus their cheap and very hard to break.
One would be able to find out more information about m4 airsoft guns online on Amazon. Two other sites that have some information about m4 airsoft guns are wikiHow and HobbyTron.
G&G sportlines ares m4 king arms m4 echo 1 guns aps guns jg guns those nice cyma ak's on airsoft megastore
The stocks generally found on m4 type airsoft guns have a level on the bottom, that when compressed, allows the stock to move.
Most airsoft guns have the motors in the grip (like AK, M4/M16, and G36 models) but some guns (mainly bulpups) have the motors entering the gearbox from the back.
there is a great selection of airsoft guns at HobbyTron.com for good prices
24.5 inches
Walmart Black Ops M4
There are many easily upgradable airsoft guns. The easiest to upgrade are M4/M16 and AK style guns, and other guns that use the same gearboxes. For example the Ares Tar-21 uses an M4/M16 style gearbox with some slight changes, but it is compatible with M4/M16 parts. Depending on how much money you plan to spend, you have a few options. You can go straight for the standard and get a Tokyo Marui, which is what all other airsoft guns are based on. Most after market parts are made for Tokyo Marui guns. If you are on a tighter budget, you can get a china clone like a JG or DBOYS with a Marui compatible gearbox. These are very good starter to mid level guns that can be upgraded with Marui parts. The only problem that I personally have with China clones is that the externals feel cheap, which I can't stand.
in my opinion kwa does very good overall kwausa.com
It depends on which companies make the m4 you have, but generally companies stick to the same dimentions for magazines for the m4/m16 family.
m4 there are spare parts
I would recommend a JG, or an Echo-1. Echo-1 guns are the same as JG's but they go through quality control and have a warranty. They can compete against higher quality band guns but are cheap, around $120-$140 ish. -id suggest an mp5 or an m16(m4) nice airsoft guns to start off with.