In a small bowl, whisk together the olive oil, red wine vinegar, chili powder, cumin, salt, black pepper, and garlic powder, then set aside.
In a large bowl, combine the cooked and cooled rice, black beans, pinto beans, corn, red bell pepper, scallions, and 2 tablespoons cilantro. Pour in the dressing and gently toss to coat.
Adjust seasoning to taste, sprinkle with the remaining 2 tablespoons cilantro, and chill in the refrigerator until ready to serve.