Cannondale have one of the best reputations as far as alu frames go.
They regularly pass a "famous" fatigue test performed by a German engineering company, and the CAAD 3 and 4 both passed back in 2001:
http://www.efbe.de/erenn.htm (scroll down)

If you're buying a 2nd hand alu frame, check closely for cracks. Cracks aren't always obvious, and often only show as suspicious paint movement.

check out Road Bike Review for old reviews. I'm not sure what year the CAAD 3s and 4s are, so perhaps start here:
http://www.roadbikereview.com/Search...ction=Products

Reviews of Cannondales are strange: some people find them as hard as nails, yet others will say the exact same bike is a smooth as butter