Yes, there is going to be a 5th book in the Twilight Saga. As mentioned in the previous post, Stephanie Meyer began working on book five in the series which is to be called "Midnight Sun", however she stopped work on the book after portions of it was leaked without her permission.
But, have no fear, as of June 2008, she has decided to continue working on the project. Due to the overwhelming number of calls and emails from fans, Meyers decided that she would in fact release the book. According to her website, there is no specific release date. If her past works are any indication of how great the next book will be, I am sure that the wait for "Midnight Sun" is going to be well worth the wait.