Just had that situation a month ago, and unfortunately according to most appraisers, the tax assessor wins out. However, if the extra room has a closet and a bed in it, sometimes the appraiser won't check. Best thing to do is get a mixture of comps with 3 and 4 BRS, price the house realistically and the appraiser will adjust accordingly.
Good idea to run a combination - but if/when push comes to shove, the tax assessor's view will win unless you can prove otherwise. The presence of a window and closet does not always do the trick if the room is below grade to any degree - with the exception of a bi-level home, where that bedroom on the lower level does count so long as it has that closet and egress. This is all my opinion - check with your agent and the township - you can also ask your agent to chat with an appraiser.
Good luck and best,
Unwavering Commitment to Service, Unsurpassed Results
It will depend if the assessor is correct or the agent is correct, a bedroom needs to be above ground, not in a basement. Have its on door, can not enter through another BR for instance, must have a full window and have heat and a closet. if this is is then its a BR if not use 3 bedrooms.