I notice dmesg is reporting this as an OpenBSD partition ("sdb4: <openbsd: >"). To mount an OpenBSD partition you need to use the ufstype=44bsd. For example:
mount -t ufs -o ro,ufstype=44bsd /dev/sdb4 $HOME/mnt
The above was tested on Linux 4.3.3-2-ARCH. I don't have an Ubuntu machine to test this on.
I notice dmesg is reporting this as an OpenBSD partition ("sdb4: <openbsd: >"). To mount an OpenBSD partition you need to use the ufstype=44bsd. For example:
mount -t ufs -o ro,ufstype=44bsd /dev/sdb4 $HOME/mnt
The above was tested on Linux 4.3.3-2-ARCH. I don't have an Ubuntu machine to test this on.