Not hitting is, as Gnu told you, a big blunder. You are correct to say that generally speaking in this type of situation you want to avoid sending a fourth checker back, because it gives your opponent the chance to make a second anchor. Here though you pay a big price if you eschew the hit, because it leaves one of your men still on the ace point and it gives your opponent the opportunity to make a five prime with his next six or make his bar with an ace. It also allows him to play all of his next roll, notably 6-6, 3-3 and 6-3 his disaster rolls if he is on the bar, are now very bad for you. The swing on these four numbers alone is enormous. Even if he does make a 1-2 back game next turn, well that isn't very bad and certainly not as bad as the alternative.