I'm blocking the diagonal threat from X that runs from bottom-left to top-right. My opponent has three Xs in a diagonal line at positions (5,1), (4,2), and (3,3). If they were to play in column 4, they would complete a diagonal Connect Four. By playing in column 4 myself, my piece will fall to row 2, blocking this winning diagonal. While this doesn't create an immediate win for me, it prevents an immediate loss and gives me better strategic positioning with three Os in column 4.