I know this is ancient and you'll actually get it by accident but if you haven't, wait till you get to the 2nd island (NE side) and find a story quest called Father's Burden (you can get the $1500 poker winnings trophy here as well). Play through the quest, which is in 3 parts. When you get to the third part you'll be told to retrieve something off a merc. He's on the side of a hill with 3 of his buddies all stood in a circle. As long as they don't see you, just fire an RPG or GL round into the middle of them to guarantee the 4 kills.