Note that although the F-4 did not initially have an internal gun, the F-4E variant did acquire an internal M61 20mm gattling cannon and a few kills were made with this gun during the Vietnam War. Also, earlier Phantoms could be equipped with an M61 in a gun pod slung under the jet, and some kills were made with those as well.

From what I understand, these guns were desired by pilots but the results in practice were mixed. And apparently ground crews hated dealing with the gun pods.