Actually AA now flys to other west coastish destinations from Terminal 3. Back a few months ago when they only had gates 32 & 33 (old QQ gates) and occasionally gate 36, Terminal 3 only served SJC & SFO. Now, they have expanded Terminal 3 operations to include many other of their "expanded West COast flights" and now have usage of gates 37 & 38.

However, AA generally has an attendant at the check in counter area in Terminal 3 who will approach you as you walk up to the line and ask you what's your destination and direct you to Terminal 4 if needed. Not much good I guess if you are not checking luggage and go straight to the gate. BEst bet is to call AA customer service and ask from which terminal you will be departing.