He is quite small and easily recognizable due to the red police light atop his body. GO-4 is equipped with two salute-ready arms and an anti-gravity stasis beam. For communication, he had various beeps and other sounds, similar to R2-D2. GO-4 units also can deliver coffee to superiors.
GO-4 used his personal elevator system to move fast around the Axiom. The entrances/exits of the elevator were usually at Steward Kiosks, the places where SECUR-T (Steward bots) were waiting inactive until they were needed. The elevator also granted GO-4 an access to the captain's quarter.
GO-4 serves aboard the Axiom as the Captain B. McCrea's first mate, and helped Auto in his attempt to carry out Directive A-113. He was accidentally destroyed when he attempted to assist AUTO in taking down Captain B. McCrea and was kicked and sent flying through a window and crashing down near one of the swimming pools in the Lido Deck.
One also appears in the video game, but it doesn't break.
GO-4 is a pun on how personal assistants are often referred to as "gophers," a verbal pun as they are often sent to "Go for (GO-4) things."
When GO-4 is sent flying out of the window, he makes a whistling sound and a smash sound very similar to the sound Wile E. Coyote makes when falling off a cliff in Looney Tunes.
According to the Pixarpedia, GO-4, like Auto, isn't really bad, he was just following orders, specifically secret directive A113. Since he was built and programmed by Buy-n-Large, he doesn't know any other way and is unable to disobey orders or have time to think whether what he does is the right thing to do due to how serious he takes his job.
Attempting to destroy the plant by putting it in the escape pod