Talmud for Shevuot 1:1

שְׁבוּעוֹת שְׁתַּיִם שֶׁהֵן אַרְבַּע, יְדִיעוֹת הַטֻּמְאָה שְׁתַּיִם שֶׁהֵן אַרְבַּע, יְצִיאוֹת הַשַּׁבָּת שְׁתַּיִם שֶׁהֵן אַרְבַּע, מַרְאוֹת נְגָעִים שְׁנַיִם שֶׁהֵם אַרְבָּעָה:

Oaths are two [i.e., Two are explicitly written in the Torah, viz. (Leviticus 5:4): "Or if a soul swear, to pronounce with the lips, to do ill or to do good": "I shall eat" — to do good; "I shall not eat" — to do ill, to deprive himself. The same applies with all things to be done or not to be done in the future, "to do ill or to do good" connoting the future.], which are four. [Another two, which are not written, are to be added: having done or not done in the past, e.g., "I ate" (when he did not eat); "I did not eat" (when he did eat)]. Awarenesses of uncleanliness [(Leviticus 5:2): "Or if a soul touch any unclean thing," in respect to defiling the sanctuary and its sanctified objects] are two [Two are explicitly written (Ibid.): "And it be hidden from him, and he is unclean," the implication being that his uncleanliness is "hidden" from him, for which reason he eats sanctified flesh — one; or enters the sanctuary — two], which are four. [Another two liabilities are to be added: "hiddenness" of sanctified flesh, and "hiddenness" of the sanctuary, when he is aware that he is unclean.] The yetzioth (forbidden acts of carrying) of Shabbath are two, which are four. [Carrying from the private to the public domain is derived from (Exodus 36:6): "And Moses commanded and they passed a call through the camp, saying, etc." — Do not carry out from the private to the public domain. ("two":) one, for the man standing outside, who puts his hand inside, takes an object, and places it outside; and one for the man standing inside, who takes an object from its place and puts it outside. ("which are four":) There are to be added another two for placing inside, even though they are not written. For just as Scripture interdicted (transfer from) one domain to another by way of carrying out, so it interdicted placing within; one, for a man standing inside, stretching his hand outside, taking an object, and bringing it inside, and one for a man standing outside, taking an object from its place, and putting it down inside.] Observations of plague-spots are two [Two are explicitly written (Leviticus 13:2): "se'eth" and "bahereth"], which are four [the toldah (derivative) of se'eth and the toldah of bahereth, which are not explicitly written.]

Jerusalem Talmud Shabbat