It really depends on the customs of your town. Take a look at this post:
You'll see how different agents have different opinions.
My suggestion is to have your agent run comps for both 3 bedroom and 4 bedroom homes. If there is a big price difference, I'd avoid calling it a 4 bedroom if you are buying it and state that in your lower priced offer. If there isn't a big difference in price, go with how you would use this room.