The other realtors are right. If it has a closet it is a bedroom. If you have your home on the market you will want the extra bedroom in the listing. You may lose a prospective buyer looking for that fourth bedroom.
If your fourth bedroom is a room without a closet (i.e. an armoire is in place for clothes storage), this may be called a 'bonus room' or something similar, but is not technically a bedroom. If this is not the case for your four bedroom home, and obviously an error, you must contact your realtor to change this for you. If your home is not for sale or rent, and you are referring to town records, you may leave this alone, at least until your house is on the market.
Is your home for sale? or is it public record data that you are using? If it for sale contact you rREALTOR to make the change. If it is public record data, your best bet is to leave it alone. If you make a lot of noise to get it changed, you will see an increase in your tax basis which means you will pay more taxes.