I've upload a 4K video every single day for the last 2 years, so I'm pretty familiar with how it works.
There aren't any settings - Just upload it as a .mp4 - Convert footage if needed to .mp4 using the h246 or h265 codecs, and upload it.
Youtube will do the rest. Once its uploaded it begins processing. It processes 360p, 1080p & 4K at the same time. Once the SD has processed, you can add cards, annotation, endings etc and publish.
The HD & 4K will be available automatically when ready. An 18/20 min 4k video, (18GB) once uploaded usually takes about half an hour for the SD version to process, and a further 70/90 mins for both the high res versions.
TL:DR - Just upload in the best res you have - Youtube does the rest without you needed to do anything, other than wait
Pro Tip: If you are wanting to get your video live ASAP, but don't wanna make just the low res available straight away (So viewers don't think - This isnt HD!) instead of hitting 'Publish' hit 'Schedule' which will automatically set it to go live the following day at midnight / 12am - Then close your browser tab, go and do something else for an hour and a half, and when you come back its finished processing - Just change 'Scheduled' to 'Live' and its done.