The 4.0 release will be sort of a new Gift Aid extension and a breaking change.

There will be an upgrade path but it will use a different extension key (internal name) so there won't be an upgrade button. i.e. you'll need to apply some understanding and care to move to v4.

For details of what's planned in v4 (and why it's felt necessary) please see https://lab.civicrm.org/extensions/ukgiftaid/-/blob/artfulrobot-v4/docs/releasenotes.md