This response BLOWS MY MIND. It's as if the user does not know the meaning of the acronym, "L-M-M-S".You should look for answers in a LINUX forum. Surely there must be a fix for LINUX users with 4K+ screens.
Visit a forum specific for your distro, and post a specific question about scaling-programs on 4K+ screens. How does other linux-programs behave?
I'm searching for a 4K GUI scaling as well... On Linux.
EDIT: What worked for me is simply to upgrade LMMS. I was on 1.1.3 (installed via apt) And I switched to 1.2.0 (downloaded AppImage from lmms.io)
In version 1.2.0, GUI scales gracefully with no tweaks.