4x100m Relay is an Aquatics Event that appears in the Wii version of Mario & Sonic at the Olympic Games. The aim of the event is for each swimmer to complete their 100m stretch in the fastest time, with the event taking place in the National Aquatics Center. The starting world record for this event is 3:12.46, while the starting Olympic record is 3:13.17. Espio appears as a referee in this event.
In this event, the player starts by choosing a team of four characters in the order that they will swim in the event. At the start of the event, as the word "Ready" is displayed onscreen, the player can press and hold the B button to charge up power, which will give them a greater speed boost and a faster starting speed the longer the button is held for. The charge level speed is indicated on a meter in the top left of the screen throughout the event. When "Go!" appears, the player must swing the Wii Remote down to start the event. Doing so with good timing will give the player an extra speed boost, however doing it too early will cause the character to stumble and perform a later and slower start. Once the character has entered the pool, the player must swing the Wii Remote and Nunchuk in a specific motion to swim, with each character having a different stroke and motion:
The faster the player swings, the faster the character swims, but swimming at higher speeds will deplete the stamina meter, indicated by a heart. If the meter is fully depleted, the character will tread water at a much lower speed until enough of the meter is refilled to continue swimming again. To avoid this, the player can press the B Button when the stamina meter is low to refill their stamina. The closer the meter is to being empty the meter is when the player presses the B button, the more stamina is refilled and less speed lost. This is also shown by the number of musical note icons that appear above the character's head after the refill, with one blue one indicating poor timing, two green ones representing good timing and three red ones representing the best. When the character reaches the end of the pool, the player can swing the Wii Remote down to gain a small speed boost from turning. When the character reaches the end of their 100m, the player can also swing the Wii Remote down to cause the next character to gain a small speed boost when diving in.