This is just rumor:
Jerry Bruckheimer would like to make a Pirates 4 with Depp that focuses primarily on Captain Jack's further exploits. Thought Johnny Depp's schedule at present looks a little full. I was in a thread on the Rumors board that discussed it. One of the posters has a cousin on the new Bruckheimer film and it is something he (JB) has spoken of as well as Depp.