That's a tough call. There probably is a "better" answer for you, but I've seen similar situations where the right answer would be to take over one of the bedrooms . . . and other situations where the better answer would be to keep it as a 4 bedroom. It depends, in large part, on what potential buyers are looking for in Wilmington.
Is there any possibility of extending the back wall--adding on maybe 5 feet to that area of the house--to expand the bath? I've seen this done a lot, especially with Cape Cods. Or extending the bath into unused attic space? Or maybe expanding the bathroom slightly, taking some space away from one of the bedrooms. Or--if the upstairs bath only opens at the moment onto a hallway--creating a second opening into a bedroom, making that bedroom the "master bedroom." Then possibly installing an area on the facing wall, perhaps with a double sink, or some other bathroom-like amenities--so that the master bedroom not only has direct access to the bath, but there's some additional items (like a vanity) on the side of the bedroom, thus making the bath more functional.
So: Check with a Realtor on what buyers expect in Wilmington. Your Realtor may be able to give you some suggestions like the ones I mentioned above. Or you might want to get a contractor in there (after getting feedback from a Realtor) to price out a few different options.
Hope that helps.