Mix the soy sauce, rice vinegar, sake and sugar in a bowl. Stir until the sugar has melted. Put the eggs in a ziplock bag, pour the marinade over and close the bag (leave some air in it). Refrigerate for about 2-3 hours and move the eggs around every 30-40 minutes, to coat/pickle evenly.
Serve with mayo, hot mustard, pickles, tarako, or with a salad. The options are endless!