The YF-4 Lightning, also known as the VF-X-4, was the prototype of the VF-4 Lightning Advanced Veritech Fighter.
The YF-4 was in service at least by 2015. In that year Captain Rick Hunter took the YF-4 into battle after Wolf Squadron was ambushed by rebel Zentraedi. Despite a transformation malfunction that prevented the YF-4 from changing into guardian mode Hunter was easily able to destroy two MAC-II Monsters. Following the incident Doctor Emil Lang chastised Hunter for risking the valuable prototype. (Comic: "Robotech: From the Stars 0: Promises")
Designed in the final years of the First Robotech War, the YF-4 was meant to replace the older VF-1 Valkyrie. However, the fighter proved to be too costly and complex and did not enter mass production.