Room count, bedroom count, and total living area all contributes

http://nationalappraiserroster.com/Resources/Appraising101/UnderstandingMeasurementsRoomCounts/tabid/223/Default.aspx

Even if your "new" room doesn't count as a bedroom, it will be counted as a "room" and considered value-added.

My problem with this is that your county tax assessment clearly labeled your home as 3-bedroom.  Even if you added a door to make a 4th bedroom, even if that new room is by all definition a real bedroom, the county record is not changed.  Ask your agent how this affect the appraisal.