Again, it's a different context. If you're in a 4th and 15 hole on a drive, then it's because your offense put you in a hole so obviously you're not going to be confident about going for it.
i think teams would opt to punt in those situations as often as they do now. 4th and 15 is not something that people tend to try and pass their way out of.
i'm sure chip kelly or someone of his ilk that prefers to take chances in those situations might try more punt fakes or passing for it, but for the most part it would be avoided much like it is today.
This would be after a score. It's not a 4th down earned via the defense stopping you. It's not the same type of gamble.