1. In a bowl, combine the mirin, soy sauce, sake and sugar, mixing well until sugar is dissolved.
2. Coat the chicken fillets well with the teriyaki sauce.
3. Heat olive oil in a wok. Add the coated chicken and cook for 3-4 minutes on each side, then remove from wok. cover and set aside in warm place.
4. Reduce the heat to low, add the remaining teriyaki sauce to the wok and simmer until reduced and thickened.
5. Remove from heat. Cut warm chicken fillets into thick slices.