When is assassins creed 4 coming out

When they announce it. There's no tangible information on another installment in the main series so far.

Assassins creed 4 is called assassins creed revelations and it's release date is November 2011. revelations is the final chapter of Ezio Auditore's story.

Edit: The Roman numerals used for Assassin's Creed I, II, and the to-be-released III should not be confused with titles like Assassin's Creed: Revelations and Assassin's Creed: Brotherhood. The lineage is as follows.

Assassin's Creed
Assassin's Creed: Bloodlines
Assassin's Creed II
Assassin's Creed: Brotherhood
Assassin's Creed: Revelations
Assassin's Creed III

I state again, Assassin's Creed III is TO-BE-RELEASED November 2012, but this is the current lineage. There may be an Assassin's Creed IV but that is only here say.