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. (From the Stars: 0: Promises)