Four people engaged in sex acts at the same time. When three of the four are of the same gender, you have a gangbang. If you have two couples (two men, two women), it´s also named as round bed.

when you're hungover as shit after a spending a night drunk as a skunk

(N) Even better than a threesome.

A sex act involving four people, in which everyone is engaging in sexual acts with each other. Two males and two females together in which the males are not participating in sex acts together can NOT be called a true foursome, it would be considered a swap.

My favorite type is when a female and three males have sex at the same time. This happens by Boy 1 giving Girl an anal, Boy 2 giving Girl regular sex, and Girl giving Boy 3 a blowjob.

Four people having sex, preferably with 3 women and 1 man, or all women.

a four-some is when 2 couples;man & woman,man & man,woman & woman; have sex next to each other and then decide to switch partners,or they can not either works.