Ubisoft has yet to reveal the setting of the next Assassin's Creed. However, a collection of fan art shows that if Assassin's Creed 4 is set in China, the results could look spectacular.
The art imagines an Assassin's Creed adventure set in ancient China. The images were made by ChaoyuanXu, a DeviantArt member. His profile page identifies him as Yuan, a freelance artist who lives in China.
The Assassin's Creed series has typically moved forward in time with every new game. The first Assassin's Creed was set in the 12th century, while Assassin's Creed 2 and its direct sequels took place in the Renaissance Era. Assassin's Creed 3, released this fall, played out over the mid-18th century. An Assassin's Creed game in ancient China would obviously predate all of these other titles. It seems unlikely that this would be a setting for Assassin's Creed 4, then.
Ancient China would be a good setting for a prequel or some other separate story in the AC universe, though. It's not a setting often explored by video games. Gamers are usually treated to a fantasy version of the country (World of Warcraft: Mists of Pandaria, Jade Empire). I think it's about time that we get to see the real thing.
In December, a Ubisoft executive indicated that AC4 would be set in Brazil. The company itself hasn't confirmed this, though. AC3 was released a mere two months ago so they may not have decided yet. If you're passionate about a particular setting, I'd suggest you follow ChaoyuanXu's lead and show everyone how awesome your idea could be.