Go here:
http://www.ar15.com/ or here:
http://www.m4carbine.net/forum.php
Read all the sticky posts at the top of the forum categories. It will take some time. You will learn a lot. You will know much more about what's important. And you won't let yourself get suckered into buying a bling gun that sits in the closet because it's actually no fun to shoot, just looks cool.
The important thing to know is that a government issue gun is a bundle of compromise. Government guns are also burst/full auto, which is a severe use and erodes the barrel throat, needing more protection.
Military specification gets talked about. They are not state of the art, they are institutionalized, and are mostly to protect the taxpayer by setting minimum standards. A lot of it is inspection and material type, and how it's to be made. Spec for a M4 states chrome lined, what barrel steel, what twist rate, etc. And, it only shoots 2 MOA, or about a two inch group at 100 yards. That's the standard, has been since before the '50s, and that's all the ammo is held to. A soldier can shoot accurately at man sized targets out to 500m depending on their skill. For shooting prairie dogs, not good enough. That's a ten inch spread and it's obvious you could miss.
A purpose built prairie dog gun might have a 20" stainless bull barrel and get 1/2 MOA, or about a 2 1/2" spread at 500 yards. A 200% improvement, and still no absolute guarantee.
Understand what milspec gets you and decide if it's good enough. It's not for a lot of shooters. Depends on what you want done. That's why it's important to read up and make good decisions, rather than stumble into it and discover something isn't what you really want.