Montreal is the lead studio.
Update: In addition to the studios listed in the Ubisoft Q&A, two further studios - Kiev and Montpelier - were included in a dev list at the Assassin's Creed 4 reveal event last week.
Original story: A team of six studios are working on the newly announced Assassin's Creed 4: Black Flag, Ubisoft has revealed.
In a Q&A released by Ubisoft, creative director Jean Guesdon explained the collaborative effort: "Ubisoft Montreal is the lead studio for ACIVBF but it's a real co-production with other studios (Ubisoft Singapore, Ubisoft Sofia, Ubisoft Québec for the Single Player and Ubisoft Annecy and Bucharest for the Multiplayer part)."
He added that this multiple studio development approach is now the norm for Assassin's Creed.
"We are extremely efficient with this kind of process now. Multiple studios have been involved in parallel on AC games starting since AC2.
"We've put in place a dedicated team to ensure the best possible communication with tools and processes to make sure that everybody knows what he has to deliver."
Assassin's Creed 4 will be released November 1, 2013.
Source: Ubisoft released Q&A