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.

In the same skillet, add ground beef and cook over medium heat until browned, breaking up meat, about 5 minutes. Drain excess fat.

Add onions and bell pepper; sauté until softened, about 4 minutes.

Stir in ketchup, brown sugar, barbecue sauce, mustard, vinegar, and chili powder. Season with salt and pepper.

Transfer the beef mixture and crumbled bacon to a slow cooker. Add all four beans; stir to combine.

Cook on low for 4 hours (or on high for 2 hours), stirring halfway through to blend flavors.

Adjust seasoning and serve warm, garnished with extra chopped bacon or green onions if desired.