These are the standard extensions that you can get from hobby city. I have them in 6S, 5S, and 4S. Length is 20cm. Quality is low, they are what they are, but if you need an extension these can be just what the doctor ordered. If you have a big job save yourself some dough and order them directly from Hobby City in bags of 10.
4S JST-XH-4 (Hard Pack)