The four corners region encompasses the corners of four different states, Northeastern Arizona, Northwestern New Mexico, Southwestern Colorado, and Southeastern Utah. There are many different reasons why this area is so unique, one being that it is the only area where four different states meet in one point, and the only geographic location in the whole United States. There are many different National Parks and monuments here to choose from, and each state has at least four different ones. One of the biggest, that everyone knows about is the Four Corners Monument, in Arizona. This marks the intersection of the four states, and is located on highway 160, 40 miles Southwest of Cortez, Colorado and the Colorado Plateau.
There are many other points of interest in this area, from archeological dig sights that you can actually visit, to ancient pueblo cliff dwellings, as well as many more. Navajo and Hopi tribes call this area their home today, but Native American history here goes back thousands of years, with several indigenous tribes coming here, and disappearing.
You can visit this area many different ways, but perhaps the best is on your own. It is a good idea to get a well detailed map, and rent a good 4 wheel drive vehicle. Be sure to bring good camping gear, and some protection from the snakes and scorpions that also call this area home. Some excellent sites of interest for any traveler to this area are Mesa Verde National Park, Hovenweep National Monument, and Petrified Forest National Park.
I hope you got a few ideas to check out in the USA.