Which of the following musical bands is not a quartet?

That's really a 'modern talking'

And behind ABBA were the 10 or so session musicians who made up the band.

My few favourites came from Modern Talking all of which are listed in the description.

I thought Geronimo’s Cadillac was by a US singer . Can’t recall whether it was Michael Murphy or Martin Mull . Haven’t heard it for about 40 years

Aww....never heard of them! And I love 80's music 🤗