Coming Soon talked to Oren Aviv (Disney's head of production) about Pirates 4 (which presumably, based on the end of the last film, will follow Capt. Jack and Capt. Barbossa as they try to find the Fountain of Youth.)
This is what Aviv said:
We’re going to shoot ‘Pirates 4′ in April or May of next year,” Aviv told us during our interview. “We are going to release it hopefully in 2011 is the plan.
It’s important to get the story right and it’s important to me to scale it down, because we can’t get bigger. The movies have subsequently gotten bigger and bigger and very complicated and they were satisfying on so many levels obviously, but I want to kind of reboot the whole thing and bring it down to its core, its essence, just characters.