#4 - "He used to be in the mafia" is the most correct. This is my friend, Jack. He used to be in the mafia. You could say, "He used to be in a mafia" but it makes the statement feel like there are multiple mafias. If I were the listener, I would then respond, "Oh really? Which one?" John: This is my friend, Jack. He used to be in a mafia. Charles: Oh really? Which mafia? #2, "He used to be the mafia" sounds a bit sarcastic. For example, if you had a strict boss who if his/her employees broke the rules, the boss would make them pay e.g. money or punishment you could use "the mafia" as a proper noun sarcastically. John: I am so glad we have a new boss. Charles: Really? Why? John: Our old boss Jack, he used to be The Mafia. You cannot say "He used to be a mafia." I hope this helps!

"The Mafia" is the name of a Sicilian criminal organization. It is a proper name, so we capitalize it. There is also a lower-case word "mafia," for a powerful and secretive group of people within an organization. If we use "mafia" we always make it clear which organization we are talking about. 1 is incorrect, because "the Mafia" is the name of an organization. "Mafia" is a proper name, so we capitalize it. The word for a member of the Mafia is "a Mafioso." "Mafioso" is a rare word. However, you could say "he used to be a Mafioso." 2 is incorrect. A person cannot be an organization. 2 is like saying "he used to be the British Parliament or "he used to the Real Madrid." He can be _in_ the Real Madrid, but he can't _be_ the whole team. 3 is not quite correct. We can use the world "mafia," lower case, to mean "a close-knit secret organization like the Mafia." It is informal. It means "a group of people who seem to have all the power in an organization." However, it is always one specific organization. The sentence should use "the." It should always say just what organization it was, to make it clear it's not the real Mafia. I might say "I used to work for So-and-so at Initech" and someone might say "Oh, yes, I've heard about her; she was in the Initech mafia, wasn't she?" 4 is correct, except that you should capitalize the world Mafia. Since the sentence doesn't say which "mafia" it is, we assume it means the Sicilian criminal organization. If it meant something else, you would say so. Thus, "Michael Corleone was in the Mafia," or "So-and-so was in the Initech mafia." You could use "a mafia" this way: "The CEO is an MIT graduate. The MIT alumni in this company are a mafia, watch out for them!"