Y 4 was the designation of a two-seat model BTL-S3 Y-wing Starfighter that belonged to the Alliance to Restore the Republic.
Kivas repairs Luke Skywalker's Y 4
Months after the Battle of Yavin, it was transferred to Docking Bay 12 of a refueling station over Devaron, so that Rebel pilot Luke Skywalker would use it on a mission to retrieve logs of Imperial communications that were intercepted by several rebel cells along the Shipwrights' Trace. With C-3PO and R2-D2, Skywalker made his way to the Tertiary Usaita system, where the starfighter was attacked by three TIE fighters and the Razor-class frigate Kreuge's Revenge. The damage that the Y 4 received forced Skywalker to escape the confrontation back to Devaron; landing in Tikaroo, he left the ship to be repaired by Kivas. After a lengthy stay in the Temple of Eedit, Skywalker claimed the repaired ship and left the planet.