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Our new Massechta begins with a mishnah that states that oaths can be of two types: to do something, or not to do it. For example, "I will eat this bread" or "I will not eat this bread." These are the oaths explicitly mentioned in the Torah. However, one can derive the same two types of oaths as related to the past, where the person swears that he did something, or didn't do it. All four types, if made inadvertently, require the offender to bring a variable-type offering, which depends on his means.
(Here the Talmud digresses from oaths to discuss other cases of "two that are in reality four." It will not come back to oaths until after eighteen pages.)
But why study oaths right after Makkot (lashes)? Because one of the last subjects in Makkot was multiple penalties for one transgression, and "two that are four" is formulated similarly.