Gluster users - Sep 2021 - Upgrading from (old!) default Ubuntu repo package

I'm troubleshooting through some mounting issues, so I thought I'd try
to upgrade glusterfs if at all possible before proceeding. I set up
this distributed pair of servers on glusterfs a few years ago. On
Ubuntu 18.04 LTS by default, 3.13.2 seems to be as far as it goes
during system upgrades. But this is way, way out of date.

Per https://docs.gluster.org/en/latest/Upgrade-Guide/, 3.13 cannot
upgrade past that point, and upgrading to 4.x is not listed as an
option. But according to
https://docs.gluster.org/en/latest/Upgrade-Guide/upgrade-to-4.0/, it's
possible to upgrade from 3.13 to 4.0, but it has to be a replicated or
distributed replicate volume and can't be a dispersed or distributed
dispersed volume.  It also links to an offline upgrade procedure, but
I'm not certain of the potential pitfalls there.

Again, I'm running a distributed volume, which is not listed in either
of the "has to be" or "can't be" categories. I verified
that by
checking at
https://docs.gluster.org/en/v3/Administrator%20Guide/Setting%20Up%20Volumes/,
as I initially wasn't sure if it was the same as "distributed
dispersed". With that in mind, do I have any safe upgrade path, or
would it be better for me and safer for the data to troubleshoot my
existing issues on this out-of-date version?