PGH-4 is played on a 4x4 board with two players. The goal of the game is to get your 4 pieces in a row, either horizontally, vertically or diagonally. Players alternate taking turns adding one of their pieces on the board from any of the 16 edges of the board. If a piece (of either player) already exists there, it is pushed forward in the same direction, possibly pushing another piece and so on. Pieces can fall of the board this way in which case it's returned to its owner. If a piece fell of the board, a move immediately in the absolute opposite direction is forbidden.