RAID10 style 4 disk ZFS mirror install

I just did a freebsd 11 root on zfs install. Using the guided partitioning there is no option for mirrored vdevs in the style of RAID10 so I have done
an install mirroring two discs. How would I best go about attaching an additional two discs to my mirror?

I'm used to the way this would be done in Linux but do not have experience doing it with FreeBSD. When I look at the partitioning it shows more than just the zfs partition, there are boot partitions and swap so I am unsure how I should add these to the discs before I add them to the zfs mirror.

I did a GPT bios install.
In any case, you can just add two disks in mirror to an existing two disk mirrored pool. That would essentially provide an RAID10-like setup. This can actually be done after you installed FreeBSD on the first two (mirror) disks without any risk. Once added you can't remove them though.