I have a Colt light weight sporter with the thin barrel profile, standard carbine handguards, A2 carry handle. I want to put a RAS on it, is the M4 RAS the one I need?

You need RIS or FF RAS.

You can use the RAS or the RIS unless you are looking to free-float.  Then the FF-RAS or the RAS II would be what you need.

[url]http://www.quarterbore.com/kac/kacrisras.html[/url]

If you are using an A2 upper, then you can forget about the RAS II. I don't think the RAS will fit the barrel you have. Abri is right, the RIS or FF RAS is what you want. Personally, the FF RAS is worth the extra couple of bucks. Either way, you will be happy. Contact  Wes @ MSTN.BIZ he will take care of you.
Steve.

Duh, forgot he said A2...no RAS II for you.

Is the RIS sturdy enought for an Aimpoint? Will it hold zero?

The Rail Interface System (RIS) is for smaller type carbine barrels i.e. 9mm or something of that nature if you take a measurement of the barrel diameter right behind the shield of where the front parts of the handgaurds go and get a measurement of .610 then the RIS is for you, If the measurement you get is from .835 to .855 then the RAS is for you. That is the most defining difference from the RIS and the RAS the other big difference is where the set screw is located RIS being at the top front of the handgaurd and the RAS being at the top back... Hope this helps you out...