4k resolution was always problematic for me in tumbleweed, you always have
something that doesn't work. The proposed solution is to scale the display
200%, but at that point you are simply using 1920x1080 multiplied x4, thus
losing the detail of 4k - and the reason why one bought a 4k monitor: To see
the extra details, to watch 4k videos, etc etc. In other words this is *NOT* a
solution. I might as well have stayed with a 1920x1080 monitor. Anyway...

I managed to work around the issue by forcing the dpi of the fonts to something
like 192 or 196 and most apps were ok, including chromium which i mainly use. 

All these worked for a couple months until I zypper dup'ed and now some
applications honor these forced dpi and others don't. (wtf?). Chromium is now
in very small mode - almost unusable, kde konsole and other qt apps are
small-fonted, yet the workspace is ok, like the launch menu, desktop icon
fonts, etc. These seem to honor the forced dpi.

So now I take the fonts up from size 12 to 18 to start seeing things again in
apps like konsole and other qt apps and the taskbar font sizes, launchers,
desktop icons become enormous (because they respected the forced dpi in the
first place) while the apps themselves are becoming normalish again.