Just because it isn't mentioned doesn't mean it was skipped.
It wasn't just "not mentioned". There is no gap in the story where you can infer "oh, they must have consumed another cup of wine during that textual pause". No, they sang the little Hallel (which immediately preceeds the fourth cup) and "went out into the night". Christ also said He would NOT drink again right after He had the third. Did He forget and drink the fourth?