We should let the author, Stephenie Meyer, answer this question:
According to a post on her web site, she was planning a fifth book in the series, which she titled "Midnight Sun." However, before she had finished writing the manuscript, someone stole a copy and uploaded to the Internet. Here is what she says about the theft:
I did not want my readers to experience Midnight Sun before it was completed, edited and published. I think it is important for everybody to understand that what happened was a huge violation of my rights as an author, not to mention me as a human being.
So what are her plans for the manuscript? Here's what she says about that:
So where does this leave Midnight Sun? My first feeling was that there was no way to continue. Writing isn't like math; in math, two plus two always equals four no matter what your mood is like. With writing, the way you feel changes everything. If I tried to write Midnight Sun now, in my current frame of mind, James would probably win and all the Cullens would die, which wouldn't dovetail too well with the original story. In any case, I feel too sad about what has happened to continue working on Midnight Sun, and so it is on hold indefinitely.
I'd rather my fans not read this version of Midnight Sun. It was only an incomplete draft; the writing is messy and flawed and full of mistakes. But how do I comment on this violation without driving more people to look for the illegal posting? It has taken me a while to decide how and if I could respond. But to end the confusion, I've decided to make the draft available here....This way, my readers don't have to feel they have to make a sacrifice to stay honest. I hope this fragment gives you further insight into Edward's head and adds a new dimension to the Twilight story. That's what inspired me to write it in the first place.
Visit her web site for the full story of what happened and the complete letter to her fans.