A hobo pack is, essentially, a foil packet containing a piece of meat and some vegetables, seasoned however you want. The meat can be a chicken thigh, or a hamburger patty, or some stew meat. It generally has potatoes and onions, sliced thin, and whatever vegetable. I like to use carrots because they hold up well in the cooking process. Wrap it all up in a heavy-duty foil packet (loosely, and make sure the edges are sealed very tight), and cook. You can cook them on a open fire (the best way) by nestling the packet into the hot coals at the edge of the fire, or on a grill (medium heat if it's gas) or in a 350 degree oven. The beauty is that cooking time is relative. On a real fire, maybe 30 minutes, a bit longer on a grill or in the oven.
Grandmother wanted to put green beans in hers, #4 was used to carrots, thus the conversation.
We have annual bonfires at my parents place, and always make these up for that. I make them on the grill in the summer, putting them together when it's convenient and cooking them later.
You can get as fancy as you want with them. I have a cookbook (License to Grill) that has a whole section on them, involving such things as lamb and rosemary, or sweet potatoes and pork, even a hawaiian version with ham and pineapple.