Just installed Ubuntu 25.04 on Virtual Box v 7.1.8 r168469

To get the full 4K resolution I had to

Increase Video Memory to 128MB

Enable 3D Acceleration

Boot Ubuntu 25.04

Manually change the resolution in the VM to 3840 x 2160

And because I want things easy to read changed the scale in the VM Display Settings to 200%

Virtual Box Display Settings

Ubuntu Display Settings