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.
Stephenie Meyer began working on the highly anticipated fifth book of the Twilight series. It is titled Midnight Sun, and it is about the events that happened during the first book but told in Edward Cullen's perspective instead of Bella Swan's. Meyer believed this would give her readers "further insight into Edward's head" and would add "a new dimension to the Twilight story."
Sadly, Meyer did not finish nor publish Midnight Sun. A draft was illegally leaked on the internet, and she became disheartened and stopped working on the novel indefinitely. Instead, she posted the unfinished version on her Web site so that her readers did not feel like dishonest if they stumbled upon the illegal leak and read it. A link to the partial draft is provided below.
I heard there is an ongoing script called Midnight Sun which Stephanie Meyer's has started but since it was leaked on the internet she has stopped working on it. There may be a chance that she will continue it since there is a lot of people who want another book .
Well there won't be extra books that will continue the saga but rather compliment it. For example, The Secret Life of Bree Tanner. Although it's an addition to the Twilight saga it does not add to the story but rather compliments the with the point of view of one of the vampires that were created for the attack on Bella in Eclipse. Midnight moon would be the same way. :D