The YF-4 Lightning, also known as the VF-X-4, was the prototype of the VF-4 Lightning Advanced Veritech Fighter. It had been a concept since at least 2014, but did not actually see creation until 2015. Some source suggest that the vehicle was abandoned due to cost, while others suggest that different models of the vehicle could still be in use through the Second Robotech War and up to the start of the Third Robotech War in 2031, although in limited numbers.
Rick Hunter owned a model of the YF-4 in January 2014, long before the first was known to be built. He played with it while considering if he should quit the military, as Lynn Minmei wanted. (Macross Saga: To the Stars)
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 Destroid Monsters.