There's piles of snowballs all around the map. The 4 bells are located in the Docks area, where you spawn. And since there's also snowballs to pick up here, you can do this trophy on round 1.
Some bells might be a little hidden at first glance: there's one in a corner behind the shack and one on a crashed little boat. There's also a bell in the middle of the area, and on the left of the ledge with two zombie spawn barricades.
It's probably easier to see visually where the bells are placed though. Here is a 30 second guide of me getting the trophy: