Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned and crispy, 10 to 12 minutes. Drain bacon slices on paper towels and crumble when cool enough to handle.
At the same time, heat another large skillet over medium-high heat. Cook and stir ground beef in the hot skillet until browned and crumbly, 5 to 7 minutes. Drain and discard grease.
Place bacon, ground beef, onion, ketchup, and brown sugar in a slow cooker; mix well. Add baked beans, Great Northern beans, pinto beans, and pork and beans.
Cook on Low, stirring often, about 4 hours.
