How do I change display scaling dynamically?

I have two monitors on my system, a 4k monitor (built-in) and a 1080p monitor (external). Obviously, this causes some issues with scaling due to the fact that windows often have weird sizes and are just in general weird.

I've been able to resolve this issue (partially) by setting my 4k monitor to downscale to 1080p (using 1x scaling), but it reverts to 4k when I unplug my other monitor. Usually, this would be awesome, but it doesn't change my scaling factor to my preferred 2x.

Is there a way I can configure my system to drop my builtin display to 1080p @ 1x scaling when I have my external monitor connected, and re-increase it to 4k @ 2x scaling when there's no other monitor plugged in?

To explain the underlying issue better (in case this is interpreted as an X/Y problem -- which it is), setting my laptop monitor to 4k @ 2x scaling makes things look good on it, but any window I open on it and drag to my 1080p monitor is double-size. For example, the Unity control center is about as big as my 1080p screen in this setup. This is obviously really bad, forcing this half-solution.

If there's a way I can keep my other monitor at 4k @ 2x scaling while keeping windows normal sized, I'd definitely appreciate a solution like that a lot more, but I think this is an X/GTK restriction, forcing the workaround I'm currently using. I've asked this question here if anyone wants to take a grab at that.