Well, I think that it would actually work if marketed correctly. Remember the 4-door Thunderbird?
While the Mustang is milestone car, there is nothing wrong with variations on the theme. The Mustang styling is strong enough to carry itself into other variants easily. I beleive that Ford could increase sales. I do not believe that there is anything sacred about a 2 door coupe bodystyle. It is largely inconvenient and cramped. The only thing that I would ask is that they never make an SUV or put the mustang name on a FWD sedan. That would be sacrelige.
I would consider a Mustang 4-door, even though I hate Fords in general. They would need to make it so that the 4-door didn't sound like you're in the inside of a drum when you start it. I find the Mustang GT and even the 6 cyl models to be unrefined loud and obnoxious. In spite of the reliability questions ,material and quality issues, and Fords lousy delaer/service network, , the Mustang's overall packaging is excellent enough to get me to consider looking Ford's way again.
I have heard, however, that Ford actually doesn't plan to do this. Nice trial baloon though.