Final Cut Pro X for Mac or Adobe Premiere Pro CC for Windows or Mac. But just because you get the software, doesn't mean your computer hardware can handle playback of 4K video.
Davinci is quite slow at 4k footage and decoding H.264 in general (a lot slower than FCPX) - but it really shines if you convert to optimized media first.
I have tested almost all windows software which claims to be able to handle 4K BUT ONLY Adobe Premiere Pro could encode it correctly. But I still use Machete to cut the original in small scenes which I import in Premiere Pro then.