Line 4 (French: Ligne 4), also known as the Airport Link (French: Antenne de l'Aéroport), is a diesel light rail line on the O-Train network in Ottawa, Ontario. The line provides a connection between Line 2 (also known as the Trillium Line) at South Keys station to Ottawa Macdonald–Cartier International Airport. The four-kilometre (2.5 mi) line opened on January 6, 2025.
