4K video editing

Ron Zamir wrote:

Complete beginner question:

I have to edit video in which I have to mix 4K and 1080 HD clips and save it as 4K.

Not a problem, people mix-n-match different resolutions all the time

What will happen to the 1080 HD part of it? How it will look?

Depends on the editing program and how/how well it decides to "up-rez"/process the 1080 to fit the 4k frame.  Some editing programs might have fancier algorithms that can make HD look nearly as good at 4k.  Others might just merely double up the lines/pixels.

Alternatively, if saved as 1080 HD how will the 4K parts look?

That also depends on the editing program/process.   Some editing programs might down-scale the 4k, making for super-sharp imagery, others might merely toss every other pixel, which is still ok.

Or in another way to ask it - how mixed video clips (4K and 1080 HD) should be edited together?

Well, if you know for 100% certain that you're only going to output at 1080 HD,  probably ideal to edit it all in a 1080 HD timeline, just for the sake of faster editing/processing/storage,. etc.

But most will probably recommend sticking with editing at 4k if you can, for the higher quality results in the end, particularly if down the line you want to export or share a 4k version.

As mentioned, try experimenting to see what your results and workflow are like just to see for yourself what the differences are and if you can notice the difference in the results.