Hello Thomas de Jong,

It's great to hear from you.

4 to 5 is a major version migration, that includes changes and improvements to the core database, that cannot be done while the database is online.

As such, no, you cannot perform a major version migration without downtime.

We are looking to reduce the toil of this process with a new experience in Aura, and we hope to have news on that soon.

I'll update you here when we have more information.

John