9 – Number of consecutive playoff series the Bruins have played in which the Game 4 winner won the series, with the B’s winning five of those (Game 4 has had a less predictive value in the last nine series the Caps have played, with the winner of that game going on to win six of the nine series).
Besides the streak, Game 4 is obviously critically important. If the Caps win, it’s a best-of-three with the Caps having home-ice advantage; if they lose… well, teams down 3-1 in best-of-seven series have gone 29-290 in those series.
(H/t WhoWins.com)