The transfer area in TBIT once you leave immigration and customs is *NORMALLY* staffed with an AA agent, though YMMV, if there is an agent they will check your bag. If not, you will need to go to T4 and check your bag, you however can walk back to TBIT and clear security there as at some peak hours it's quicker than T4, due to T4 construction shortening the TSA security screening area.