Shevuot 5 - Two Ways to Violate Shabbat, Which are Really Four
The Sages added two more prohibitions, where the action is done jointly. For example, if the house owner takes the object and lifts it up, and the poor person accepts it while outside, neither of them has completely violated Shabbat.
There are also four cases of bringing an object in. This makes a total of eight, and is not parallel to the "two that are four!" To answer this, we count only those cases where one is liable for a Torah violation, and equate taking out to bringing in, because they are both, in essence, changing the domain, and you will get exactly two that are four.
Art: Venetian Water Carriers by John Singer Sargent
