Since I have 6, I play the 13. I only need 2, and there are 2 down table. You might choose the safety noted below.
IMO, you only shoot the 4 when you need 3 or more, since pushing it down is nothing more than a third ball in play for your opponent. The advantage of pushing the 4 down is less than the disadvantage of leaving 3 for your opponent to score -which would even the game. IMO, you shouldn't fall for the 4 ball 2-railer. It's a potential (big time) loser for you.
And, when you are playing for the big cheese, you might even consider pushing the 13 up table 2 rails and laying the cue ball on the bottom rail, especially if you don't like the cut on the 13. This removes another ball from play (the up town game) and really works to your advantage.
-td