While the CAR A4 is a nice rifle, I would go with the only model RRA offers with a chrome lined barrel, the "Entry Tactical". With the post ban model, you get a 16" chrome lined barrel that looks like a 14.5" with flash hider due to the mock suppressor.
There is much discussion if you need chrome lining, but IMHO it is definitely easier to clean and gives you more reliability under less than ideal circumstances.
Forget the RRA quad rail- it is too heavy. Yankee Hill, the RRA rails producer, already has a lighter one out and the new SureFire rail shouldn't cost that much more. I would go with the SureFire rail- otherwise you would have to remove your front sight for installation. Or the much more costly route would be to go with a (barrel) free floating RAS II or SIR system.
Also specify that you don't want the Tactical Carry Handle- it is of no use with optics.
Most people consider the Arms#40 back up sight the way to go. While a folding front sight may be nice, it is not necessary.
A Vltor stock is great, but if you want to keep the rifle post ban, a Sully stock or the RRA "Entry" stock with a spacer (both nonfolding but with the right size = shorter than the A2 stock) is not bad too.
Best Regards
Cato
PS: In most of the recent SWAT issues Pat Rogers, a real authority when it comes to the Ar15/M4, has discussed questions which may interest you. I ordered some back issues because of them.