Numbuh Four is one of the main children from Codename: Kids Next Door as well as the shortest of the five. He is the hand-to-hand combat expert of Sector V and comes from Australia. He has two parents that always look at the bright side, and has a baby brother in his family. His truculent personality and slightly dim wits often get him tricked or suckered. But despite this, he's got a strong sense of determination and never gives up in a fight. He hates all girly things, and fears swimming. Despite this, he has a hidden crush on fellow KND operative Numbuh Three (or, at least, hidden to her). As adults, they're married.
In FusionFall, he fell when the invasion intensified in The Future. In The Past, he's at Mount Blackhead with Numbuh Three. He helps the player find and take on Fusion Tolietnator and also hands out missions. During the mission for the MacNano, he was much needed help when the player tries to recover Mac's bag from Fuse's monsters only to realize a gooby trap was set inside. His Fusion lies within the Dinosaur Pass infected zone found within the Darklands. The Fusion Numbuh Four secret lair is outside the infected zone near Really Twisted Forest.