Add soy sauce, mirin, sake, and sugar to a saucepan. Bring to a boil over medium heat and whisk until the sugar dissolves.
Once boiling, reduce heat to medium low and simmer for 8-10 minutes until reduced by about half. It should thicken slightly to a thin syrup consistency.
Remove from heat and transfer the sauce to a clean jar. Leave uncovered to cool to room temperature (it will thicken more as it cools).
Once cooled, secure the lid and store in the fridge for up to 3 weeks.