The D4 is cheap today, but it isn't the right tool for architectural as it is optimised for speed rather than for great DR. You would be better off with a D850 or, if money is tight, a D810. You want as much dynamic range as you can get, especially for real estate where you would ideally want to use strobes but there seldom is time to set them up perfectly.
If you are getting into that market professionally, remember to budget for a good wide angle zoom, a wide angle tilt/shift lens and a sturdy tripod as well as the body. Depending on your local market, a good drone and the license to use it commercially is probably necessary.