The above video runs down what went into Prust's suspension, highlighting that Prust made principle contact in the chest and head area, the hit was delivered almost a full second after Stepan released the puck, and Prust has been suspended once before for an illegal check.
Earlier today, the Rangers announced that Stepan suffered a broken jaw during Game 3, which likely played a major factor in Prust's suspension. Before the Rangers announced Stepan's injury, there was speculation the hit would draw no suspension whatsoever. No penalty was assessed on the play, which occurred 2:45 into the first period.
The incident sparked Daniel Carcillo to go after Prust later in the period. On a subsequent whistle, as Carcillo was being escorted to the penalty box by a linseman, contact he made incurred an automatic 10-game suspension by the league today.
Prust will be eligible to return this series in Game 6, if the game is necessary.