Your 1st step is to contact a qualified marketing agent that serves you area. This agent will advise you if the subject neighborhood is selling with 3 or 4 bedrooms. Typically you want to be on par with homes in your neighborhood.
Feel free to contact me, to discuss your situation. I can research the MLS and discuss the pro's and con's of your potential project.
I also have a FREE website that allows homeowners to track their value. (it also shows the number of bedrooms of homes in your neighborhood) Just click on the link to sign up.