I'm playing in column 4 to block a potential fork by my opponent. If I played elsewhere, they could place an X in column 4, row 4, creating three Xs in a row horizontally (in columns 2, 3, and 4). This would force me to block one side in their next move, but I couldn't block both sides. By playing in column 4 now, I prevent this fork while also building vertical pressure with my existing O in the bottom row.