Answer:
13 US metropolitan areas host all 4 major sports; NFL, NBA, MLB, and NHL.
Minneapolis/St. Paul, MN (Vikings, Timberwolves, Twins, Wild)
Denver, CO (Broncos, Nuggets, Rockies, Avalanche)
Phoenix, AZ (Cardinals, Suns, Diamondbacks, Coyotes)
San Francisco Bay Area, CA (49ers/Raiders, Warriors, Giants/A's, Sharks)
New York City (Giants/Jets, Nets/Knicks, Mets/Yankees, Devils/Rangers/Islanders)
Miami, FL (Dolphins, Heat, Marlins, Panthers)
Philadelphia, PA (Eagles, 76ers, Phillies, Flyers)
Boston, MA (Patriots, Celtics, Red Sox, Bruins)
Chicago, IL (Bears, Bulls, White Sox/Cubs, Blackhawks)
Atlanta, GA (Falcons, Hawks, Braves, Thrashers)
Detroit, MI (Lions, Pistons, Tigers, Red Wings)
Washington, D.C. (Redskins, Wizards, Nationals, Capitals)
Dallas, TX (Cowboys, Mavericks, Rangers, Stars)