I don't know how else to put it, so I'll just say it as simply as I can.
It seems that Roger Goodell went into his chambers, crunched the numbers and decided that four games is ample punishment for buying a couple of 20-year-old girls some alcohol.
Maybe it is, maybe it isn't. I just don't know how he figured it out. Could be he has a chart similar to the one that shows general managers how many points each draft pick is worth when weighing a potential trade.
But I suppose there must be some system.