I'm playing in column 4 to block a potential diagonal threat from my opponent. Currently, my opponent has Xs at (4,2) and (3,3), which form a diagonal pattern. If they were to play at (2,4), they would have 3 Xs in a diagonal row, creating a strong position. By placing my piece in column 4, it will fall to position (2,4) (since positions 3-5 are already filled), preventing this diagonal threat. This is primarily a defensive move to limit my opponent's options.