Your home is listed as a 4 bedroom because that is what it shows in the public records. How many bedrooms did it have when it was first built? Was the 5th bedroom added on later? Does the 5th bedroom have a permanent built-in closet? If not, then we wouldn't classify that as an actual bedroom. It would be a den or bonus room.
However, if all 5 bedrooms were there from the beginning, then it's just a mistake in the public records and it wouldn't be the first time. You could always let the County Assessor know about the 5th bedroom.