Catalan Opening: Open Defense

With 4… dxc4, the Catalan Opening: Open Defense allows White to sacrifice a pawn to enter a positionally technical game. Usually, the g2 bishop gains more vision and becomes a very strong piece. 
In the Open Defense, the game can take many different directions, with black having options like 5… a6, 5… Nc6, 5… c5, 5… Bb4+ being the most popular at the master level. Usually however, this is the starting point for many flashy imbalances and the control for an inherently unstable center.