I'm running Mint 19 Tara, with Cinnamon 3.8.9, on a Lenovo P51 with a 3840×2160 graphics display.
Most apps, such as vim, Chrome, Libre Office, and everything that came pre-installed, work correctly. But some apps, like gimp, Zoom, InSync, and the Java-based VPN client that I've had to install to connect to one of my customer's networks, have tiny GUIs. (I doubt that gimp and Zoom run on Java, though, so this is unlikely a Java-specific issue.) The fonts and icons are barely big enough to read, and they're laborious to click on because of their size. The dialogue windows themselves are also very small.
I've improved gimp by installing a new icon set, but they icons and text are still a little small, and this doesn't help with the other apps.
I don't think this issue has anything to do with Cinnamon, because changing the text scaling factor in System Settings→Fonts only affects apps that already have a good font size.
I wonder if there's some other UI/window manager that the problematic apps are using, e.g. Qt, and if there's some other system-wide setting that I can tweak to fix it. Any ideas?