For the marinade - In a medium saucepan or skillet, mix together all the pineapple juice, soy sauce, sake and mirin. Bring to a boil and simmer 1-2 minutes. Remove from heat and cool to use as is for a teriyaki marinade for beef, poultry or seafood.
To make the pineapple teriyaki sauce - bring mixture to a boil, lower heat to simmer for about 8 minutes if using a saucepan, or 5-6 minutes if using a skillet. Simmer and cook until reduced and thickened. It turns into a glossy, syrupy sauce.
Allow to cool to room temperature before using. Store in a sealed container in the refrigerator and use within 2-3 weeks.
Notes
Substitute sake with Chinese cooking wine or a dry sherry.