I'm surprised, but VirtualBox-6.1.18-porteus-v5.0-x86_64-1prt.xzm also runs on Porteus 4.0 Xfce.
How can that be?
Post here if you are a new Porteus member and you're looking for some help.
That's because you probably kept the same kernel. Vbox module is created for a given kernel version, and respecting that it should work on different Porteus versions (there's a limit of how far you can go, of course ).