A musical ensemble, also known as a music group or musical group, is a group of people who perform instrumental or vocal music, with the ensemble typically known by a distinct name. Some music ensembles consist solely of instruments, such as the jazz quartet or the orchestra. Some music ensembles consist solely of singers, such as choirs and doo wop groups. In both popular music and classical music, there are ensembles in which both instrumentalists and singers perform, such as the rock band or the Baroque chamber group for basso continuo (harpsichord and cello) and one or more singers.
Four parts
The four-piece band is the most common configuration in rock and pop music.  Another common formation was a vocalist, electric guitarist, bass guitarist, and a drummer (e.g. 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, Creed, Black Sabbath, Van Halen, Rage Against the Machine, Gym Class Heroes, The Stooges, Joy Division, and U2.) Instrumentally, these bands can be considered as trios. This format is popular with new bands, as there are only two instruments that need tuning, the melody and chords formula prevalent with their material is easy to learn, four members are commonplace to work with, the roles are clearly defined and generally are: instrumental melody line, rhythm section which plays the chords or countermelody, and vocals on top.  In some early rock bands, keyboardists were used, performing on piano (e.g. The Seeds and The Doors) with a guitarist, singer, drummer and keyboardist. Some bands will have a guitarist, bassist, drummer, and keyboard player (for example, Talking Heads, Gerry and the Pacemakers, Small Faces, King Crimson, The Guess Who, Pink Floyd, Queen, Coldplay, The Killers and Blind Faith).  Some bands will have the bassist on lead vocals, such as Thin Lizzy, The Chameleons, Skillet, Pink Floyd, Motorhead, NOFX, +44, Slayer, The All-American Rejects or even the lead guitarist, such as Death, Dire Straits, Megadeth and Creedence Clearwater Revival. Some bands, such as The Beatles, have a lead guitarist, a rhythm guitarist and a bassist that all sing lead and backing vocals, that also play keyboards regularly, as well as a drummer.

Q: what does four parts refer to?
A: The four-piece band is the most common configuration in rock and pop music.

Q: who are some examples of a four part band?
A: 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
a vocalist, electric guitarist, bass guitarist, and a drummer