Ubisoft has finally found a solution to Assassin's Creed Unity's ongoing issues and the latest patch will be "replacing and updating portions of the Paris city map where we are seeing issues".
That sounds a lot like me replacing this entire article because I can't find the spelling mistakes. Whatever's wrong with the game can't be fixed so they're just getting rid of chunks of the map and replacing them, hence the large 6.7GB size of the download. The full patch notes mention it's "fixed various collision and mesh issues", which had been mentioned before as causing issues.
However, if you're on Xbox One you'll see a 40GB download because "an issue with the patch downloading process is replacing the entire game instead of just the parts affected by the patch". So the update to fix the game is broken. Ubi are working on that but "have no ETA on when this will be resolved". If you've got the game on disc however you can reinstall it from there and it will download the correct-sized patch.
[[IMPORTANT]Patch 4 Release Notes | Forums - Ubisoft]