Conversation so far:
What does a Musical ensemble four part refer to
The four-piece band is the most common configuration in rock and pop music.
Who are some examples of a four-piece band?
The Who, The Monkees, Led Zeppelin, Queen, Ramones, Sex Pistols, Red Hot Chili Peppers, R.E.M., Blur, The Smiths, Echo and the Bunnymen, The Stone Roses are four-piece bands.

Question: what four roles are there?
Self-contained rewrite: What four roles are there in a four-piece band?