I've seen many reviewers use a syringe to test it and they got 5ml.
I understand what you are trying to tell me. As I have mentioned, I have tried to fill it for more than 4ml, but I end up eating e-juice for the first few puffs immediately after refill. So said, my feel is, if you try a default and quick refill, you will end up filling it up to about 4ml...