In explanation of this phrase, the Rambam in his Commentary to the Mishna points to the passage in Sefer Vayikra (5:4) that describes how someone who swears le-hara o le-heitiv – which is understood by the Sages to mean someone who swears to refrain from doing something or someone who swears to do something – will be liable to bring a sacrifice if he does not keep his word. Thus the “two” mentioned in the Mishna refers to either a negative or a positive oath; the “four” refers to the possibility of taking such oaths with an eye towards either the future or the past.
The presentation of this rule leads the Mishna to mention other laws that have the same “two that are four” pattern, including carrying on Shabbat from one domain to another and types of tzara’at – biblical leprosy – and how they are to be recognized. Both of these cases parallel the case of oaths in that they contain two basic concepts that include four ideas.
Although the focus of this tractate is on shevuot, the reference to tzara’at in the Mishna leads the first two perakim of the tractate to teach about a side topic. Most of these two perakim are devoted to discussions of ritual purity in the Temple, and more specifically, to arranging atonement for desecration of the purity of the Temple. The Torah demands great care with regard to maintaining the purity of the Temple, and we find many mitzvot whose purpose is to keep the Temple safe from ritual defilement. As a general rule, the main purpose of the laws of ritual purity relates to the Temple, and, as a consequence, today, when we do not have a Temple, most of those laws no longer apply.