Quatuor

'Kwatt-oo-or

'Kwatch-or

'Kwatt-roar

Those are three ways that you can -- very approximately  -- pronounce "quatuor", which is the French for quartet.

(String quartet = quatuor à cordes)

I never knew before that there are exceptions to the rule that French "qu" is pronounced like k. 

I should have known, because of "aquarelle". Usually these "kw" exceptions are in borrowed words (e.g. quatuor from Latin) and usually they come before the letter a.

A quintet in French is "quintette" and this exemplifies another unusual pronunciation of "qu", though not by all French speakers. Some pronounce it koo-ang-'tett, though many just say kang-'tett. 

In English, too, quartet and quintet demonstrate two different ways of pronouncing "qu" : at least for me and everyone I know. We pronounce quartet kor-'tet, but Google Translate gives the pronunciation kwor-'tet.

"Quartet" derives from an older French word "quartette", ultimately from Italian "quartetto". The oldest English uses seem to be in the context of vocal pieces, e.g in operas.