Yes, its a doable walk. Just exit from AS baggage claim (area C) at Terminal 3, and turn right. Terminal 4/AA will be the 2nd building on your right. It shouldnt take more than 5 mintues or so. The only obstacle would be dodging the throngs of people entering/exiting the Bradley Intl Terminal (depending on time of day).

You could also take the LAX Terminal Shuttle - Bus "A," but walking is bound to be faster.