A #4 should be plenty big enough. I have also caught them in cubby sets baited with fish. What is the biggest bodygrip you can set on land? We are lucky enough to be able to set 330's on land here in Alaska. 220's work pretty well, if you are able to set 280's I would look at those instead. You can fence otters in pretty hard, they tend not to shy away from it. I have had decent luck setting the trap upside down also on trail sets, so the trigger is on the bottom. I have found that it allows me to spread the trigger wires a bit wider creating a bigger opening but they still get caught. If you use a foothold either put it on a slide wire or make sure there are plenty of swivels, they will fight pretty hard.
