Link: HiDPI on dual 4K monitors with Linux
Vincent Bernat's article HiDPI on dual 4K monitors with Linux (via) is about what you'd expect it to be about and is, as they say, relevant to my interests. Especially relevant to me is the section on HiDPI support on Linux with X11, which runs down a collection of issues and contains a very useful chart about what is supported in what application and toolkit, which added some information that I hadn't known.
Note that Bernat's experience with xterm and rxvt don't match mine, perhaps because we're setting the X-level DPI information in somewhat different ways. My experience, as covered here, is that plain X applications using XFT fonts scale them appropriately once you get the DPI set everywhere (ie, if you tell xterm to use Monospace-12, you will get an actual 12 point size on your HiDPI monitor, not 12 points at 96 DPI and thus tiny fonts). If you use bitmap fonts, though, you're in trouble and unfortunately xterm still uses those by default for some things, like its popup menus.
(It's the nature of these articles to become out of date over time as HiDPI support improves and changes, but it's still a useful snapshot and some of these applications will probably never change.)