Add mirin, soy sauce, sake, and sugar to a small saucepan and stir to dissolve the sugar.
Bring to a gentle simmer over medium heat, then reduce the heat to maintain a steady simmer.
Cook, stirring occasionally, until the bubbles thicken and the sauce reduces by about one-third and lightly coats the back of a spoon, 8–12 minutes.
Remove from heat and let stand 5–10 minutes to thicken further.
Use as a glaze during the last minutes of grilling or broiling, or as a dipping/drizzling sauce. Cool completely before bottling and refrigerate for up to 1 month.