Four people get together for a double date. Three of these people are beginning to get uncomfortable when the fourth abruptly tells his date that he wants to show her his duck. When he excuses himself to the bathroom, the other three people decide to get themselves only 3 glasses of wine as a way of silently telling him that he is no longer welcome on this double date. When he gets back from the bathroom, he looks at the rest of his group with a confused expression and says "Do you have a fourth glass of wine?"
Other than that situation, I don't see many other appropriate times for this question.
That's because 'et' could be translated as either 'one' or 'a'.
Strictly speaking, only 'ett' can be translated as one, so 'et fjerde glass' has to be 'a fourth glass' and can not be 'one fourth glass'. But you can translate it that way if you're less strict about it. In that case, you can rule out 'one' based on context, because 'one fourth glass' doesn't make sense in Norwegian or English.