Since the 3.4 release the GitHub build system was migrated so that addons are built into OH v4.0 Jars. => Is it possible to test an OH v4.0 addon Jar on my live OH v3.4 operative system? (note: the actual addon Jar does not depend on Java v17).
No, that’s very difficult. It’s probably easier to create a branch from 3.4 release and then cherry-pick the changes to the branch and build a bundle for 3.4.