You can trivially do 4K Windows 9x (and more) under VirtualBox with the open driver @ https://www.os2museum.com/wp/windows-9x-video-minidriver-hd/
Just edit the .inf file before installation to add the resolutions you'd like, as the author recommends on the blog post.
Or after installation change HKR\System\CurrentControlSet\Services\Class\Display\0000\MODES\ to add whichever resolutions you'd like.
As per what the author says, I can see that 5,120×3,840 is the max resolution that will be shown in the control panel applet. I'm assuming you could still go larger if you just set it via API calls.
Happy to see that VBMOUSE works with it too.
Obligatory useless "5K+" screenshot attached.