Are doing the work just for the investment return? If so, you may not see your work translate into $$ for a few years, depending. If you will be able to utilize the space for a few years before selling, the project will have more value to you. It also depends on the current house, location and quality and utility of the improvements you make. So, overall, it depends...
This answer really does depend on your house. For example if your neighborhood is all 2 bedrooms houses you probably won't get much return because you are already king of the hill. If your house is in a neighborhood of houses all with 4 bedrooms, then you will be catching up to the rest and should see a better return. However, for us to quantify exactly what you would return would require a more complete picture. You may want to contact the realtor who sold you the house and ask them to run a CMA for the property with 4 bedrooms and then one with 3 bedrooms.
Good Luck on your project.