By playing in column 4, I can create a horizontal row in the bottom row, forming a sequence of three O's (columns 2, 3, and 4). This setup creates two winning threats on the next move (by dropping in column 1 to extend left or in column 5 to extend right), a typical fork that the opponent cannot block in one move. Given its centrality and the dual threats it creates, column 4 is the optimal move.