Conversation so far:
what does four parts refer to in the musical ensemble?
The four-piece band is the most common configuration in rock and pop music.
who are some examples of a four part band?
The Who, The Monkees, Led Zeppelin, Queen, Ramones, Sex Pistols, Red Hot Chili Peppers, R.E.M., Blur, The Smiths.
what four roles are there?
a vocalist, electric guitarist, bass guitarist, and a drummer.

Question: are there alternatives to that line up?
Self-contained rewrite: are there alternatives to the line up of a traditional four part musical ensemble?