Revealing the news on their Live Updates site for Unity, Ubisoft talked about how they need to refurbish the Paris map, and that is what is holding them up:
Rigorous quality control is of paramount importance to us, and your feedback over these past weeks has indicated that it is important to you as well.
As stated last week, we’d expected to release the patch today (and we know that many are anticipating its release), but we are choosing to hold off until we can give you the improvements we’ve promised. We are committed to delivering major performance improvements, which requires that we refurbish the Paris map and that will take a few more days to hit the high level of quality our players deserve.
Thank you very much for your understanding and continued patience.
When Patch 4 gets a release date, we’ll let you know.
[Source: Assassin’s Creed]