However, in another place in the Talmud where the idea of "two that are four" is discussed, only four cases of Shabbat violations are mentioned, and not eight. Here is what it says there. There are two kinds of oaths to violate: to do something in the future or not to do it. These are really four - adding oaths that he did or did not do something in the past. The list continues - two ways to remember that one was ritually impure when he went to the Temple, which is four. Two ways the spiritual leprosy (tzaraat) can look, which is four. So it is four or eight?
The Talmud answers: here in tractate Shabbat, it describes all the variations and details; thus, there are eight. But in another list, where the subject is oaths and Shabbat is only incidental, only the four major categories are discussed.
Art: Shabbat by Marc Chagall
