I saw this the other night.
If I'm not mistaken something similar happened a few years ago. When he was in the minors, BJ Upton, flipped a bat at an Umpire. And I think he got hit pretty stiff with like 20 or more games, or more, suspension.
Nevertheless, although it seemed unintentional to hit the Umpire with his helmet, he certainly intended to throw a tantrum & should subsequently be responsible for his equipment he threw.
Bumping an Umpire is one thing, but to have him hit by an object as a result of your tantrum is different.
4 games seems a bit light.