Heat the olive oil in a medium saute pan over medium heat. Add the onion and cook, stirring occasionally, until transparent and soft, about 5 minutes. Transfer the onion to the slow cooker, along with the red bell pepper, potatoes, 1 cup corn, broth, cumin, smoked paprika, cayenne pepper, and salt.
Cook on low for 8-10 hours or on high for 4-6 hours, until the potatoes are tender.
Turn the slow cooker off and remove the lid. Allow the soup to cool slightly. Using an immersion blender or working in batches with a regular blender, puree the soup. Return it to the slow cooker and turn it back on.
Stir in the remaining 3 cups corn and soy milk. Cover the slow cooker and cook on low for another 20-30 minutes, until heated through. Season with salt and pepper to taste.
Serve topped with additional corn, diced bell pepper, and/or sliced scallions.
If you're going with frozen corn, I recommend Trader Joe's fire-roasted corn. It adds an extra note of smoky flavor that can't be beat.