Metropolis is the largest city in the world, located directly in the middle of all four nations.  It is an independent, secular, democratic city state.  It is divided into four quadrants, each of which elects one person to congress.  In addition, each of the four nations sends one representative to hold a seat.  The congress deals with issues both specific to Metropolis and international relational issues.  The representatives are reappointed/reelected every 5 years.  No member can serve in consecutive terms.  Metropolis is the center of the economic, political and social world.  People from every corner of the globe can be found here, whether for business, pleasure or otherwise.  Metropolis is home to enormous banks, markets, schools, and museums.  The city center is sectioned off from the rest of the city by a gigantic wall.  The upper echelon of the world resides and does business in the central district.  It is by far the wealthiest area in the world and is guarded as such.  Some of the most powerful benders in the world are hired by the Metropolis congress to defend the wall, ensuring none but those who belong are allowed to enter.