My bet is Victorian England. The pic seems to have an industrial revolution worker class feel to it.
I really liked AC4, got me back into the series after the booring pretentious 3rd installment, although there was plenty room for improvement.
The combat is going to be odd though, considering that in the latter part of the 18th century there is no excuse arming 2/3rds of the goons with swords any more. Unless they are like hooligans or something.
Also jack the ripper. Considering his victims where members of the the assassin orders long standing allies: Whores. He might be a major antagonist.