In order to install OS/2 Warp 4 in VirtualBox using the CD download on this website you need to:
1. Set the OS as Windows 3.1
2. Select the 'Installation.img' file as the floppy image
3. Boot from floppy
4. Follow installation instructions
5. Then you can set the OS as OS/2 Warp 4
6. Enjoy OS/2!
(The reason you set it as Windows 3.1 in the beginning is because OS/2 setup generates a weird error, then freezes)
If you find another way, just reply to this message.