Combine soy sauce, garlic, ginger, sake and sugar in a bowl. Add the chicken pieces and coat evenly. Cover and refrigerate for at least 1 hour.
Add 1 inch of vegetable oil to a pot and heat until the oil reaches 350°F. While waiting for the oil to heat up, line a wire rack with paper towels and put the potato starch in a bowl.
Take the marinating chicken out of the fridge and add a handful of chicken to the potato starch. Toss and coat each piece evenly.
Fry the chicken in small batches (about 5-6 pieces at a time) until it's a medium brown and the chicken is cooked through. Transfer the chicken onto a wire rack to drain the oil. If you want the chicken a little crispier, fry the chicken a second time after it's been cooled off.
Serve with lemon wedges.