By examining the board, I noticed that dropping in column 4 gives two benefits: it blocks the potential diagonal threat from my opponent (X) along the (2,1)-(3,2)-(4,3)-(5,4) line and builds a vertical stack in column 4. Since there are already two O’s in column 4 (at row5 and row4), my drop will land at row3, creating three in a row vertically. This sets up a forced winning threat since an additional drop in column 4 (if unblocked) would win. Therefore, column 4 is the best move.