Rinse the chilies in cold running water. Trim off the stems with a pair of scissors. With a toothpick (or other sharp pointed object), make some random holes around each chili. (I typically make 6 to 8 holes per chili. This is to help with sauce absorption). Alternatively, you can chop the chilies into small pieces (about 3 cm / 1 inch) without making the holes.
Combine the soy sauce, water, vinegar and sugar in a saucepan. Bring to a gentle boil over medium-high heat, whisking occasionally, until the sugar has dissolved (about 5 minutes). Remove from the heat and let cool for about 10 minutes.
Tightly stack the chilies (without crushing them) in a glass jar suitable for pickling, then pour the warm brine over them. Seal the jar and leave it at room temperature for 24 hours. Transfer it to the fridge. For small chilies, you can start eating them after 72 hours. For larger chilies, allow about 1 week before serving.For extra Korean flavor, toss the pickled green chilies with the spicy Korean chili sauce (optional ingredients listed above) just before serving.