1) Is this a problem only with MC19, fixed in higher versions?
There have been a lot of improvements for high DPI and 4K screens since MC19, so probably.
2) Are MC22 or MC23 user interface fully functional with 4K monitor and 4K video playback?
I don't have any 4K content yet, so I am outputting from MC22 at 1080p, and having my Sony TV upscale to 4K. The TV does a great job upscaling, so I think that is best for the moment. A search of the forums is bound to find threads on setting up and using 4K TVs, and yes, people are using MC playing native 4K video to 4K TVs. But search for confirmation.
3) Even if MC22 have fixed up the user interface for 4K, some icons may still be tiny. Are there ways to make them larger?
The two setting to play with are;
1. MC Options > Theatre View > Appearance > Size
2. Windows 10, Display Settings > Scale and Layout > Change the size of text, apps, and other items.
Also for Standard View there are "View > Size" and "Option > Tree & View > Advanced > Select font". If the above options, don't do what you need, try these out.
Search for threads about High DPI screens.