The M4 carbine has a muzzle velocity of 2,970fps(884m/s).
The M4 3181 AEG is always nice. Here's a link: http://www.hobbytron.com/AirsoftAssaultRifleGunAEGWAA10CH3181.html
The M4 3181 AEG is always nice. Here's a link: http://www.hobbytron.com/AirsoftAssaultRifleGunAEGWAA10CH3181.html
375 FPS
300 fps or 320
14.5 inches on the actual M4 rifle.
It's about 350. Has very good range.
375 FPS
the lightest you can find
The selector lever. That's pretty much it.
The M4 is capable of automatic fire, the AR 15 is not
Could you define mk4? If you mean the M4 carbine, currently used by the US military, it is about 2900 fps. If you mean the .303 caliber #4Mk1 used by British forces in WW II, it is about 2441 fps.
For spring M4, you can go for WELL or Double Eagle products. If you're looking for M4 AEGs, go for JG or D-Boys. If you're looking for M4 GBBR (Gas Blow Back Rifle) go for WE. Chine made airsoft are cheap but they keep improving their quality. It also have good fps (usually more than 400 fps) out of the box for AEGs so you don't need to upgrade the internals. I hope it can help.