Red Sox magic number is 4 for what? The Wild Card? I know that the Yankees have 2 magic numbers, and it's kind of confusing.
Their magic number of 1 will clinch the AL East division title. That's all the games they would have to win. Now, technically the Red Sox COULD tie them in the W-L columns if they only won one more game. HOWEVER, with the Yanks-Red Sox season series at Yankees 10, Red Sox 9, the Yanks would win the division on that tiebreaker.
So, if the Yankees make it to their magic number of 2, with either 2 wins, or a Yanks win and Red Sox loss, or whatever combination, they would then have the best record in the AL East, and win the division without reverting to tiebreakers.
It was all very confusing to me at first, but that's the deal in the AL East.