@dusx said:

Of course backup before upgrading, since file format will be updated and won't be openable in V3 if you need to go back for any reason.

Just chiming in because, while it's technically correct, the wording of this warning makes the process sound much scarier than it actually is. The feature that's been in place since the launch of Isadora 3, which prompts you to create and update a copy of a file that was made in an older version of Isadora (leaving the original file untouched), is still in place, so it's impossible to accidentally update a file to the Isadora 4 file format by misclicking once or by opening an older file using Isadora 4 without backing it up first. There is no way to disable this safety net feature, and the default choice in the dialog is to create a copy (leaving the original file unmodified). 

TLDR: Isadora 4 is designed to preserve your original file and work with a copy instead so that you don't have to think or worry about making a backup yourself.

Best wishes,

Woland