I'm able to play back 4k mp4 files from my FZ1000 in HF3P. They're choppy, but that's due to my laptop having an older 570M GPU. I can create a proxy file that plays smooth. I haven't tried an AVCHD file, though.
HitFilm does work on PC. That's where it began, and was later ported over to the Mac. I use HitFilm 3 Pro for my compositing/FX needs, and enjoy the program. I probably won't be upgrading until after the first of the year.
I have Adobe CC, and do my editing in Premiere. HitFilm's editor is functional, but fairly basic. But I prefer HitFilm to AE. I like it's interface better. Plus, I don't have to add great particles and 3D through plugins.