The Ontario Airport Southwest Terminal operates out of Terminal 4 at ONT, which is the main terminal building. The airport has two terminals, Terminal 2 and Terminal 4, but Southwest Airlines flights depart from and arrive at Terminal 4.

Terminal 4 at Ontario International Airport offers a range of facilities and services for passengers, including check-in counters, security checkpoints, baggage claim areas, dining options, and retail shops. It's always a good idea to check with the airline or the airport's official website for any updates or specific details regarding your flight or terminal information.