The 4 x 400 metres relay or long relay is an athletics track event in which teams comprise four runners who each complete 400 metres or one lap. It is traditionally the final event of a track meet. At top class events, the first 500 metres is run in lanes. Start lines are thus staggered over a greater distance than in an individual 400 metres race; the runners then typically move to the inside of the track.

Relay race runners typically carry a relay baton which they must transfer between teammates. Runners have a 20 m box (usually marked with blue lines) in which to transfer the baton. The first transfer is made within the staggered lane lines; for the second and third transfers, runners typically line up across the track despite the fact that runners are usually running in line on the inside of the track. This prevents confusion and collisions during transfer. Unlike the 4 x 100 m relay, runners in the 4 x 400 typically look back and grasp the baton from the incoming runner, due to the fatigue of the incoming runner, and the wider margins allowed by the longer distance of the race. Consequently, disqualification is rare.