What happens to the items in a bag of holding if you slice it open? We had an instance where a player character placed a bag of holding over an enemies head and sliced it open.
The rules are silent on this.
There are not (to the best of my knowledge) standard rules for the non-magical destruction of magic items in 4e. Additionally (and unlike D&D 3.5), the Bag of Holding entry says nothing about how to break it, or what happens in that circumstance.
So either magic items are not casually destructible (which is my guess), or this is a gap they left in the rules.
I'd lean toward "magic items aren't easy to break," for two reasons.
First, the previous editions did emphasize the destruction of the Bag of Holding, to the point where it became a common joke/meme. Because of this, 4e's silence on the issue is unlikely to be an oversight.
Second, 4e's ethos does not look kindly of the casual elimination of character power and agency.
So what happens to the objects inside a Bag of Holding if it is somehow broken?
That's up to the individual group to decide; there are no explicit rules to follow.
In a certain campaign, there was a cult dedicated to a chaotic god from the Outer planes. There was a specific ritual that demanded you put 10,000gp worth of treasure inside a bag of holding and then slice it open.
The items stored inside were then lost in a strange dimension as an offering to the god.