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1.
Definition:

A type of slap using the back of one's hand. First, the hand is cocked at a 45 degree angle across one's body and above one's head, tucking the thumb into the inner hand and extending all four fingers. The slap is then released across the face of the receiving party.
Often, one warns the receiver first with the hand in the cocked position: "Do you want a fresh one?" By "fresh one," "fresh four" is what is implied.
by Sweaty Electric September 21, 2004