Will there be a 4th installment to the Pirates of the Caribbean franchise?

It's entirely possible, though it would be without Knightley playing Elizabeth Swann.

From Wiki:

Walt Disney Pictures has reportedly made tentative plans to shoot a fourth film in 2009. Johnny Depp has stated that he would love to play Jack Sparrow for another trilogy "if they had a good script." In February 2007, after filming was completed on Pirates of the Caribbean: At World's End, screenwriter Terry Rossio said he and Ted Elliott may try to write a screenplay, though "It's like the fourth Indiana Jones picture 鈥?there are forces in play to make it happen, and forces in play to make it not happen."

Producer Jerry Bruckheimer has indicated interest in a spin-off, but was exhausted by the trilogy's production. Gore Verbinski feels that "the big danger is diminishing the brand... it would have to be a tale worthy of telling" for him to return. If the director returned, he noted that "I would start fresh and focus on the further adventures of Captain Jack Sparrow." Keira Knightley has stated that she does not wish to continue playing her character of Elizabeth Swann as she wants to do different projects, since she has been involved in the filming since she was 17 years old.

No i doubt it, it would be good if they did.

Probably. They rarely stop making sequels until every ounce of life has been drained out of a plot. Sometimes not even then.

Haven't we all suffered enough?

We can only hope not. They all suck.

First off, to all of those who don't like POTC, buzz off! We fans don't want to hear your negative attitude.
Now to answer your question, there most likely will not be fourth installment in the Pirates of the Caribbean franchise. If you waited to watch the clip after the credits, it implies that it is the final installment.

GOD I hope not!!!!!!