As far as quality goes, I would rate them in this order:
1. Colt
2. Bushmaster
3. Olympic
If I were you and wanted a good deal but still have a decent chance of getting a good rifle, you might also consider the Rock River Arms and D.P.M.S. versions.
Let me also say that I hate the post-ban muzzle brakes that come on the Colt, Bushy and Olympic. They are noisy, look horrible and IMO, have to go. If you get one of these send it down to Kurt's Custom and get him to remove that brake and install a fake flash hider or one of his A2 style brakes. They look much better and are also much less noisy. Or you could just get a model that doesn't have any brake at all and not have to worry about it.
And do you really even want an M4? If you want it because it looks cool or because it will offer you some performance advantages (i.e. compatible with various optics such as Aimpoint, EOTech, ACOG, lights, etc) then by all means get one. But M4's seem on average to be priced higher than a flat-top AR carbine that doesn't use the M4 type barrel, handguards and buttstock. Think about this because it may save you money and still give you a good gun that's just as adequate for your purposes. A good 16" Colt Sporter with lightweight 1:7 twist barrel and A2 buttstock is about perfect for me. Of course others will have different preferences.
Another low cost option for you is to buy a kit and build your own gun. I would recommend the J&T kits, as they seem to be good quality. You can check them out at [url]www.jtdistributing.com[/url]. You can get models with the features you want and leave out the rest. Plus, by doing this, you get to learn a great deal about how your weapon works.
Oh, and if it were me, I would get one that uses a 1:7 twist rate if at all possible. It will give you the best range of options when it comes to ammo. Slower twists may not stabilize some of the newer heavy match ammo. This ammo could come in handy should you ever want to use your gun for long range, accurate shooting or for home defense, since it also happens to perform well in that regard. Good luck in your hunt!
-Charging Handle