24 isn't choppy, its the most normal looking to the eye. 60fps viewed on a screen with a high enough refresh rate looks more fluid like a home video... almost looks worse. That's subjective though I guess. Most productions are filmed at double the FPS the final product will use, so that they have the freedom to do slow-mo without loss.
4K at 15 fps is pointless. It's a spec acheived purely for marketing benefit if you ask me. And even if it got 4K at 24fps, with the IQ of the GoPro, you'd be better off scaling up the resolution of DSLR video taken at 720p. Id be willing to bet the IQ will still be as good or better.
4K = 9.something megapixels by the way.