Neko's point about a large battery in the stock is another thing to look for. Some M4s have the battery in the front hand guard, while others have it in the stock. (Front wired vs rear wired, respectively). I prefer my battery in my stock, but it doesn't make a whole lot of difference to me. You may find that if you have a front wired gun, your battery won't fit in the hand guard, so you have to use a "battery box" (a little box that goes on your front rails).
Large batteries aren't much of an issue anymore, because there's LiPo batteries now, that are small, and hold a lot of power. (Do a little research before you buy one though. They can be dangerous if mishandled, and can die if not charged/discharged correctly).
So yeah, front vs rear wiring is something that you may want to keep in mind. (Not a huge deal though)