I don't have the card on me so I'm going to make up some numbers. For the rat swarm (and other swarm cards), instead of the usual two states of defeated and undefeated based on the check to defeat number, here there are three states: undefeated, defeated but shuffled back in the location, and defeated.
Let's pretend the check to defeat is a Combat 8 (which I think it is, but don't remember for certain). If your total roll is under 8, you take combat damage just like you would for losing against any other monster. If your roll is 12 or above, you defeat it just like any other monster and it gets banished to the box. The new part here is when you roll 8-11. It is still defeated since you got the 8 (so you don't take damage), but you did not defeat it by at least 4 (8+4=12), so instead of going back to the box it gets shuffled back in the location deck.
skizzerz numbers were exactly correct.
Check to defeat: Combat 8
If you do not defeat the Rat Swarm by at least 4, shuffle it into the deck it came from; the Rat Swarm still counts as defeated.
If your dice result is 9, 10, or 11; the Rat Swarm counts as defeated, but you shuffle it back into the deck it came from instead of banishing it.
We actually had that come up in an OP scenario. Amaryllis had to defeat a random monster to close her location while Seelah (at the very least an honorary cleric, in our estimation) dealt with the villain. Pulling a Rat Swarm, Amaryllis handily rolled a total of 8.
Still counts as defeated though~