For $1000 you can get a Colt 6920 - essentially their "Law Enforcement" edition. Similar to what the Military M4 runs. Usually around $950 online, $1050 in shops. Some Walmarts are also reportedly carrying them.
Colt, Novoske, BCM, LMT.
Middle road brands? (probably still good enough)
Rock River Arms, Stag, Smith & Wesson, PSA (only b/c they're new)
Not so well thought of brands?
DPMS, some of the Bushmaster models (some are OK).
You might be able to go cheap by buying a used upper (the part that goes bang) and a cheap lower, or building one yourself (that's the part w/ the trigger and buttstock).
Go to Ar15.com and m4carbine.net and do some reading. Make sure you read the stickies in the sections you're interested in. Then set aside at least $400 for a case of ammo and 5 mags.