Combine vinegar, water, sugar and salt in a saucepan and bring to a simmer, stirring to dissolve sugar. Simmer for 1 minute, then pour into a large, spotlessly clean glass jar and let cool completely.
Slice green chillies across so they are slightly less than 0.5-cm (¼-in) thick. Place sliced chillies in a heatproof bowl, pour over boiling water to cover completely, let stand 10 seconds, then drain well. Let cool.
Add chillies to jar of pickling solution. There should be enough liquid to cover chillies. If not, top up with a bit more vinegar. Cover tightly and keep refrigerated.
If only touched with clean spoons and stored refrigerated, pickled green chillies will keep indefinitely. If the liquid starts to look cloudy or the flavour of the chillies starts to fade, chuck them out and make a fresh batch.