Probably inspired by the success of Chevy’s four-door Monza, Ford apparently gave serious consideration to a four-door Mustang. Probably just as well that Ford gave it a pass, but that’s not to say the work on it was wasted, as it clearly inspired another Ford sedan.
There’s a lot of four-door Mustang in the 1966 Falcon. But based on the rather weak sales of the ’66 and up Falcon, it didn’t do it much good. But the Australians loved it.