I tried to do this via xrandr + selecting 2× scaling from the UI, with limited success for three reasons:

1) The display preferences panel did not know about my custom backing framebuffer and kept re-setting it to match the native resolution on each click (could be resolved by careful ordering of operations)

2) The mouse cursor became tiny (could be set to bigger sizes, although it looked like you are picking an unreasonably huge mouse size on the control panel)

3) Couldn't break out of the original 4k bounds with the mouse

...and that's were I left it last time.