In a small saucepan, bring the soy sauce, mirin, sake, and sugar to a boil over medium heat. Simmer for 1 minute. Allow to cool to room temperature.
Bring about 2 inches of water to a boil in a medium pot. Gently lower the eggs and reduce to a simmer. Cook for 9 ½ minutes for jammy eggs with strong whites.
Immediately drain the water and place the pot under running cold tap water for 1-2 minutes until the eggs have cooled. Peel the eggs and place them in a Ziplock bag.
Pour the marinade into the bag. Try to get out as much air as possible then seal the bag. Refrigerate for at least 4 hours. Consume within 3 days. Enjoy!