Terminal 1 used to be the International Terminal. When United took over all of T1, they made a temporary "terminal" in the basement of the parking garage, with shuttle busses to the airplanes. That was T4. When they built the new international terminal, therefore, they called it T5. The concourses, however, are labeled sequentially (H/K/L in T3, M in T5).

I'm not sure I'd be up for a gathering in T4, as that is now the area for valet parking and oil changes...