cool, that's a great price!  If you want to get an M4 upper, unless you want a RAS from KAC, make sure you get the M4 handguard, it's doubel shielded and thicker.

The Rail Interface System has white alpha numerical "addresses" on top, bottom, left and right to assist in remounting an accessory.  There are others products that mimic the RIS but they don't have the feature above.  The Army has chosen it, because it's useful and also probably because KAC was the first to come out with it, there was no competition.  The M4 RIS cost close to $300, http://www.jtsurplus.com  It's versatile, has reportedly better cooling than even the M4 handguard, and is the "real thing" not an imitation.  In my opinion, unless you want to rig your M4 like an Urban Warrior http://www.uws.com/UrbanWarrior.html or need to mount tons of stuff on it http://www.securityarms.com/20010122/galleryfiles/799.htm , the flat top of the upper receiver should do.
I'm not in the services or an LEO, Bear_Hunter can defintely give you his professional opinion on it (RLTW!) hehe