Your home is not on the market. Trulia receives their data from public records, which don't currently reflect the 4th bedroom.
Sites like this and Zillow are only as good as the data they access. If you want these sites to more accurately reflect your homes characteristics, you might want to update King County Records. Of course this could have a tax consequence, so proceed carefully.
No algorithm or formula can reliably interpret the real facts of the market, both in terms of what a home actually offers in room count and condition to where the buying public is reacting to.
If you are looking to obtain a more reliable valuation, feel free to contact me or most local agents who work in the area and tour the homes regularly. We'd love the chance to help.