Add the soy sauce, sake, mirin and sugar to a sauce pan over medium low to medium heat. Without bringing it to a boil, stir with a spoon spoon until the sugar completely dissolves. Remove from heat.
Pour into a storing jar and keep in the refrigerator for up to 3 months.
Once the teriyaki sauce mixture comes to a boil in step 1 above, reduce medium low heat and simmer for 10-15 minutes, or until thickened to your liking.
