The term 'PART OF A CHESS SET' can be broken down into two solutions: ROOK (4 letters) and PAWN (4 letters). The ROOK is a castle-shaped piece that moves in straight lines, either horizontally or vertically. It is the most powerful piece on the board and is used to protect the King. The PAWN is the weakest piece on the board and moves one square at a time, either forward or diagonally. It is used to capture other pieces and can be promoted to a higher rank if it reaches the opposite side of the board.