Meanwhile mix yogurt, garlic, and salt and pepper to taste.
When rice is almost done, place a 10- to 12-inch frying pan over high heat. When pan is hot, add oil and spinach. Stir until leaves wilt, about 2 minutes. Mix in shrimp, remove from heat, and drain liquid from pan.
Enclose tortillas in microwave-safe plastic wrap and heat in a microwave oven at full power (100%) until hot and steamy, 45 to 60 seconds.
Onto each warm tortilla, spoon 1/4 of the rice, 1/4 of the spinach mixture, 1 tablespoon yogurt sauce, and 2 tablespoons salsa. Fold one side of the tortilla over the filling, then fold the sides over and roll to enclose. Accompany the burritos with remaining yogurt sauce and tomato salsa, to add to taste.