> In terms of "what I expected to happen" - well, I expected an actual 4K resolution (like 3840x2160) to work. Maybe not by default, but at least when I enable it, it should be able to auto-scale the appropriate parts of the system and application UI in a consistent manner.
That's what it does. OS X will by default render at the panel's native resolution. But it scales everything to a lower effective resolution. Say you're running a 5K monitor at "2560x1440" (as in the iMac 5K). And the app displays a 32x32 icon at (0,0). The OS will see that you're running in 2x mode (real resolution = 2x effective resolution). So it'll pull a 64x64 version of the icon from the app's art assets. It'll take up 32x32 on the virtual 2560x1440 display, but a 64x64 image will be what's copied to the frame buffer. Text glyphs, and images, likewise, will be rendered at full resolution.
When you bypass the default setup and tell the OS to run at 3840x2160, you're not telling it to render at that resolution. You're telling it to size things as if you're running on a standard definition monitor of that size. So when the app displays a 32x32 icon, the OS will see you're running at 1x mode, and pull a 32x32 icon from the art assets. The result will be a tiny icon.
That's what it does. OS X will by default render at the panel's native resolution. But it scales everything to a lower effective resolution. Say you're running a 5K monitor at "2560x1440" (as in the iMac 5K). And the app displays a 32x32 icon at (0,0). The OS will see that you're running in 2x mode (real resolution = 2x effective resolution). So it'll pull a 64x64 version of the icon from the app's art assets. It'll take up 32x32 on the virtual 2560x1440 display, but a 64x64 image will be what's copied to the frame buffer. Text glyphs, and images, likewise, will be rendered at full resolution.
When you bypass the default setup and tell the OS to run at 3840x2160, you're not telling it to render at that resolution. You're telling it to size things as if you're running on a standard definition monitor of that size. So when the app displays a 32x32 icon, the OS will see you're running at 1x mode, and pull a 32x32 icon from the art assets. The result will be a tiny icon.