1. In a mixing bowl, whisk together soy sauce, mirin, sake, and sugar until the sugar is dissolved.
2. Heat the vegetable oil in a skillet over medium heat.
3. Add the chicken thighs to the skillet and cook until browned on both sides, about 5-7 minutes per side.
4. Pour the teriyaki sauce over the chicken and bring to a simmer. Cook until the sauce is thickened and the chicken is cooked through, about 10 minutes.
5. Serve the teriyaki chicken over rice or with your favorite side dish.