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.
are there alternatives to that line up?
In some early rock bands, keyboardists were used, performing on piano (e.g.
The Seeds and The Doors) with a guitarist.
what is a hallmark of this type of set up?
Instrumentally, these bands can be considered as trios.
This format is popular with new bands, as there are only two instruments that.
that have what?
two instruments that need tuning, the melody and chords formula prevalent with their material is easy to learn.
is there any other set ups?
Some bands will have the bassist on lead vocals.
are there any band examples of this type ?
Thin Lizzy, The Chameleons, Skillet, Pink Floyd, Motorhead, NOFX, +44, Slayer.
where there any others?
The All-American Rejects or even the lead guitarist, such as Death, Dire Straits, Megadeth and Creedence Clearwater Revival.

Question: what other instrument can be switched out for another?
Self-contained rewrite: Besides bass guitar, electric guitar, drums and vocals, what  instrument can be switched out for another?