For me, it would depend on what I use the 4th bedroom for.
An office? Well, I can buy $100 7x7 bookcase/room divider from Ikea to divide up the space a little more, or have the office in my bedroom. ((Honestly, I'm biased, having lived for 3 years in a house that just isn't *quite* big enough, the extra space is a HUGE bonus for me, and I'd much prefer to do some creative decorating to have BREATHING space.)) If the house has a garage, it would cost less than a new fridge/appliances to outfit THAT as a living space and park outside.
However... If it's Naptime for daycare kids &/or other space for the daycare kids to keep it separate from the rest of the house, and there's no "dividing a space" or garage option? Probably the 4th bedroom.