1. Soak skewers in a bowl of water until ready to use. In a large bowl, combine 2 tbsp each of the soy sauce, mirin and rice vinegar and 1 tsp of the sugar. Stir in chicken pieces, cover and marinate in refrigerator 30 minutes.
2. In a small saucepan over medium-high heat, add remaining 2 tbsp each soy sauce, mirin and rice vinegar, and remaining 1 tsp sugar. Stir to combine. Bring to a boil. Simmer 3 minutes, until thickened. Divide glaze between 2 small bowls.
3. Heat grill or grill pan to medium-high. Thread skewers, alternating 1 to 2 pieces of chicken with 1 piece of scallion. (Makes 8 skewers.) Lightly coat grill or grill pan with oil or cooking spray. Grill skewers 3 minutes, brushing tops with glaze in a bowl. Flip skewers and brush with glaze from second bowl. Grill 3 more minutes, until chicken is cooked through.