I'm placing my piece in column 4 to block my opponent's horizontal threat. They currently have 'X's at positions (2,4) and (3,4), and if they place at column 4 next, they would create a dangerous three-in-a-row situation. Additionally, this move connects with my existing 'O' at (4,5), potentially setting up a vertical opportunity for me in the future. It's a defensive move that also maintains some offensive potential.