I have a paperback version of The Hammond World Atlas (1989) which was a "gift" with a Newsweek subscription, and which has large scale maps of each of the United States: I have found in it a specific reference to the "Only point in the United States common to four state boundaries", which are Utah, Colorado, Arizona and New Mexico: it is in a Navajo Indian Reservation and Google Earth tells me that its coordinates are 37 degrees North, 109 degrees 2´40" West. Roger & out.