Read here. This article articulates very clearly why going for it more on 4th down just makes sense. I think there is a good reason so many fans and avid watchers of the game want to go for it on 4th down – math supports it, and punting often feels like an overreaction to the situation (especially on 4th and short). This is why I like that McCarthy isn’t afraid to fake punts, do onside kicks, fake FGs etc. He could, however, go for it on 4th down more.
As I’ve noted before, Gregg Easterbrook, ESPN’s Tuesday Morning Quarterback, has also long been a proponent of this. And it’s not surprising that 2 of the most successful coaches of the last decade or so (Belichick and Payton) are subscribers to this theory.