Have you looked at the new PRO model? I know there is a 5 year difference in the battery life, but other than that, it should be pretty tough/reliable for half the money, right? I've had two Aimpoints in the past and I was very happy with them. Does the M4 come with lens caps like the other ones?
Yes, I bought new Pro's for my SWAT team. You can get a Pro for $380-$400ish. You can find a used Comp M4 for $500ish. I would buy a used Comp M4 over a new Pro. However, any Aimpoint will serve you well.
I currently use a couple T1's, Comp M4 and a Pro.