How can you be with someone for four years an not have picked up on anything she's into?
What does she do in her spare time? Work out? Read? What kind of TV shows does she watch? How is her sense of humor? Does she collect anything? How is her computer? Her cell phone? Has she chosen a career? If so, what?
You should be able to answer questions like these after 4 years, and these are some things you may want to consider when purchasing her gift. If she has a bunch of books, check out who the authors are and find something similar. Maybe she'd like some DVD's of a TV show that is no longer on air that she loved. If she has an older cell phone, a new one may be in order.
If she won't tell you want she wants, feel free to get her something you know she needs (like dishes or new tires, etc) and explain you're no good with the guessing games and if she wants something specific that isn't a practical gift, she just needs to come out and say it.
People who expect others to read their mind are always so surprised when they don't get what they want, and I have no idea why.