Question:  I’m slightly confused sorry, you said it’s not possible to capture 4k
on OS X but the MacBook Air has sufficient hardware for 4k. Does that
mean it’s just the operating system that’s the problem and if I ran
windows or Linux on my MacBook Air I could do 4k?

If I were to just capture 4:2:2 1080p on my MacBook Air what codecs
could I capture in? pro res for example? Or does this depend on the
capture software?

Answer:  4K capture is not possible on OSX because it is related to OSX. Using
your Mac with Windows/Linux to capture 4K will be possible.
You can of course use your Mac to capture 1080p with QuickTime//ffmpeg
under OSX because the 4:2:2 format is supported under OSX. You can also
use ProRes.

in 4K2USB3Capture / Input