A foursome is a group of four people who do something together — dinner, tennis, a walk, a card game. Dictionaries often paint it as two men and two women, and that picture is common, but it is not a rule: four friends of any mix can still be a foursome. In golf it is the usual word for four players in one group; some golf games also use it in a tighter way (two pairs sharing a ball). The word feels friendly and everyday. Though actually, in informal English a “foursome” can also mean four people having sex together, the same shape as “threesome.” In class and in polite talk, the first meaning — four people going out or playing a sport — is the one you want. If you only mean “four people,” “a group of four” never sounds awkward.