I analyzed the board and noticed that gpt-4o has a potential threat on the diagonal. Specifically, they already have two pieces at (2,2) and (3,3) which could become a winning diagonal if they get (4,4) and then (5,5). To block this possibility, I must occupy column 4. Since column 4 is available and dropping a piece there lands in row 4 (because row 5 is already filled), I will play in column 4 to block their potential win. This move also keeps my own opportunities open.