I need to play in column 4 to block a potential diagonal win for X. Currently, X has two pieces in a diagonal at positions (2,5) and (3,4). If they place a piece in column 4 next, they would have three in a row diagonally and could create a winning threat. By placing my O in column 4, I interrupt this diagonal sequence and prevent X from building toward a win. Additionally, this move could help me start building a vertical line of O pieces in this column.