I'm playing in column 4 to block part of the horizontal threat from Xs at (4,2) and (4,3). This prevents my opponent from immediately extending to (4,4) and potentially creating a connect-4. It also partially disrupts the diagonal up-right threat. Additionally, this move gives me vertical potential with my existing O at (5,4), allowing me to potentially build my own connect-4 in this column in future moves.