Put the soy sauce, sake, sugar and ginger in a large Ziploc bag and mix together. Add the chicken thighs and make sure each piece is coated with the marinade. Press as much air out of the bag as possible, then seal and refrigerate overnight.
The next day, tear off a 3 foot long sheet of aluminum foil with the shiny side up. Lay half the marinated chicken thighs on the foil, skin-side down, to form a rectangle about 6 inches wide. Sprinkle the chicken generously with white pepper. Lay two chicken tenders on the chicken thighs. Since tenders taper on one end, you want to lay them facing in opposite directions so you end up with a core of chicken tenders that is roughly the same thickness of both ends.
Tightly roll the chicken being careful not to sandwich the foil between the layers of chicken. When the chicken portion is rolled, continue rolling with the rest of the foil to form a tight cylinder. Twist both ends of the foil shut.
Cut an 8 foot length of twine, fold the twine in half, then tuck the midpoint under one end of the roll. Bring both ends of the twine up either side of the roll, cris-cross the twine, then flip the roll over and bring it back around. Repeat until you've reached the other end of the roll, then tie the twine in a knot. The twine should look like a laced shoe on either side. Make a second roll using the rest of the chicken.
Bring a steamer to a boil, then add the rolled chicken. Cover with a lid and steam for 1 hour. When it's done, remove the chicken from the steamer and allow it to cool. Place the roll in the refrigerator overnight to let the collagen set.
When your chicken chashu is ready, unwrap it, slice it and put it on a bowl of chicken ramen, or have it on top of rice as a donburi.