These links provide all documented changes to Geant4 and bug reports, these should be the first port of call

Main download page (As of writing this directs to Geant4.11, which has not yet been tested by myself); and

GitHub source

Previous releases (As of writing directs to Geant4.10.6 as the first page, bottom of the page contains pages for each previous version)

• Here you can find a quick link for:
  • Release Notes (For Geant4.10.6, can easily change the version in the link to 4.10. for version number instead of following above steps)
  • Patch Notes (Also for Geant4.10.6, patch 3. As before "Patch4.10.6-3" can be changed to "Patch4.10.-**" for version number and patch number **)

User Forums (Forums with easy to search/navigate topics. Search here for some key words and see if what you're looking for has been discussed, resolved or taken further)

Bug Report (Mostly actual bugs and issues that were recommended for flagging. Sometimes these are not present in the forums)

User Support (Varying guides and examples for the use of Geant4, most links on this page link to most recent Geant4, 11.0 as of writing)

• All of the newest documentation can be searched and navigated here
• Previous documentations found here (split into individual guides)

As mentioned, despite all this it can be difficult to scour through these links to find what you're looking for, and even when you have exhausted it all sometimes the issue or topic you're looking for may not be documented at all. Some changes in Geant4 are silent, 'minor' changes based on empirical evidence, which may be detrimental to experiments that look in other directions.