Bring 3 cups of chicken stock to a boil. Add brown rice to stock when boiling. Reduce to a simmer and cook for 30 minutes.
While waiting for stock to boil, chop peppers, onion, celery, and tomatoes into 1/4" pieces. Mince garlic.
Heat butter in large skillet over medium heat. When butter is melted, add onion, garlic, peppers, and celery (not tomatoes). Cook until vegetables are beginning to soften, approximately 5 minutes.
Cut chicken thighs into roughly 1/2" cubes. Add to vegetables and cook for an additional 5 minutes.
Pour remaining 1 cup of stock and chopped tomatoes into vegetable and chicken mixture. Stir, making sure to loosen and mix in any vegetables sticking to the pan.
Add seasonings (salt, pepper, paprika, thyme, oregano, bay leaf) to chicken and vegetable mixture. Stir in rice and stock mixture. Cover and cook for an additional 10-15 minutes until rice is tender and chicken is cooked through.
Serve as is, or with a little finely chopped green onion or parsley on top, if you prefer.