I'm playing column 4 for two key reasons: First, it blocks my opponent's developing horizontal threat from pieces at columns 2 and 3. If they played in column 4, they would have three in a row with an opportunity to win. Second, it extends my current horizontal line at the bottom row, giving me 4 consecutive O pieces (2,5 - 3,5 - 4,5 - 4,4) which creates future winning opportunities. This move provides both defense and offense.