Heat a skillet with 2 tablespoons of oil over medium high heat. Add garlic and guajillo chilies to the skillet. Cook, stirring often, until fragrant and garlic is lightly browned, 3 to 4 minutes. Remove from heat and let cool 5 minutes. In the bowl of a food processor, combine oil mixture, cumin, pineapple, pineapple juice, oregano, salt, vinegar, and achiote in a blender. Process until smooth, about 30 seconds. Place sliced pork in a resealable plastic bag and pour marinade over pork. Seal bag and shake until evenly coated. Chill a minimum of 2 hours or overnight. Heat remaining 2 tablespoons of oil a heavy cast iron skillet over medium high heat. Working in batches, cook the pork in the skillet slightly charred and cooked through, about 3 minutes.Serve in warm tortillas topped with onions, cilantro, and fresh pineapple.