By now you have seen this bedroom question is a gray area. My suggestion is to sit down with your Realtor and show him/her the answers you have gotten to your question. Go over the responses and then decide what's the best avenue for you in marketing your home.
I would suppose that if the city of Keene calls the house a 3 bedroom on the tax card, then that is what it is. And because the tax card is usually a document reviewed in the transaction by the buyer, it would be smart to use that information in the listing. On the other hand, if the tax card says you have 4 bedrooms and you really have 3, tell the Assessor and get an adjustment--you've been paying too much tax.
If you are located in an area of the city that does not have town water and sewer--meaning that you have a septic system, then the ability of the septic to process water will determine how many bedrooms you can legally have. Usually the state of NH determines that there would be a maximum of 2 people per bedroom, and each person would average 60 gallons of water a day for all uses, washing, toilet, dishwasher etc.
A septic that can handle 360 gallons a day would be required for a 3 bedroom home. ( 6 people x 60 gallons). Sometimes the data on a septic is not available--but generally something is known and is recorded on the field tax card at the city hall. It doesn't matter that you use one room as an office and there are only 2 people living in the home...this is how the state does it. (See no closets involved).
Good luck with your sale.