Rinse the chillies in cold running water. Trim off the chilli stems with a pair of scissors. With a toothpick (or other sharp pointed object), make some random holes around each chilli. (I typically make 6 to 8 holes per chilli. This is to help with sauce absorption). Alternatively, you can chop the chillies into small pieces (about 3 cm/1 inch) without making the holes.
Combine the soy sauce, water, vinegar and sugar in a saucepan and boil until the sugar dissolves (about 5 mins) on medium high heat. Whisk occasionally. Remove the pan from the heat and cool it down (about 10 mins).
Tightly stack the chillies (without crushing them) in a glass jar suitable for pickling then pour the brine over them. Seal the lid and leave it at room temperature for 24 hrs. Transfer the jar to the fridge. For small sized chillies, you can start eating them 72 hrs after making them. For larger sized chillies, you can start eating them one week after making them. If you want some extra Korean flavour, mix the pickled green chillies with the spicy Korean chilli sauce (optional ingredients mentioned above) just before serving.