A Four-Man Band, despite assumptions, is not simply a smaller version of the Five-Man Band archetype. While the Five-Man Band have skills just crying out for an action or an adventure story, the Four-Man Band is a primarily comedic group, designed for a sitcom or domcom. The four distinct personalities are specifically designed to play off each other, and Hilarity Ensues.
Please note that in a Four-Man Band there is noHero. The focus is evenly spread between them. If there is a Main Character (rare) they can easily be any one of the Band, or it can just be switched between episodes to give focus to different characters.
Only Sane Man - Or maybe just the guy who comes the closest to being sane most often. Every zany gang must have a straight man, and the other three leads have to share one. While his (or her, although the role is usually male) friends act out, the Normal Guy's role is to Lampshade and be embarassed by their behaviour. He will be more successful with the opposite sex than his friends, notably The Pervert.
The Smart Guy - More intelligent than his friends. His Five-Man Band equivalent can do research for the group or explain some of the tougher concepts to the rest of the band or the audience, but this variant is primarily geared towards Lampshading the relative stupidity for the others and being humiliated for his arrogance. Alternatively he may be a Mad Scientist type, with insane inventions.
The Butt Monkey - A character who serves as a target for the others' jokes. There are a few tropes that could make the Band's Butt Monkey; The Ditz, the Extreme Doormat or others. This member has remarkably less charisma than his friends or is a better target for jokes. The others will pick on him frequently and make him the target of practical jokes. Sometimes, however, this character may get one over on his bandmates, and this will thoroughly humiliate the others.