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.

Michael Fred Phelps (born June 30, 1985) is an American swimmer who has, overall, won 16 Olympic medals—six gold and two bronze at Athens in 2004, and eight gold at Beijing in 2008, becoming the most successful athlete at both of these Olympic Games editions. In doing so he has twice equaled the record eight medals of any type at a single Olympics achieved by Soviet gymnast Alexander Dityatin at the 1980 Moscow Summer Games. His five gold medals in individual events tied the single Games record set by compatriot Eric Heiden in the 1980 Winter Olympics and equaled by Vitaly Scherbo at the 1992 Summer Games. Phelps holds the record for the most gold medals won in a single Olympics; his eight at the 2008 Beijing Games surpassed American swimmer Mark Spitz's seven-gold performance at Munich in 1972. Phelps' Olympic medal total is second only to the 18 Soviet gymnast Larisa Latynina won over three Olympics, including nine gold. Phelps also holds the all-time record for most gold Olympic medals, at 14.