What are the differences in terms of stability? Is it safe to do a rolling upgrade?
I don't know much about the stability, they did patch a bug related to rolling updates from 3 to 4 tho.
The Mariadb website has some additional information:
It should work with rolling upgrades, but as always, be careful and have a local backup ready in case something goes wrong. Galera isn't very beginner friendly and it takes knowledge to manage a whole cluster without issues.
Its said to be very few differences, tho you might want to start from scratch. My cluster crashed once I tried to upgrade from 3 to 4. Maybe I did something wrong.
Samuel_Oldfield What does the error logs say?