How many players in a quartet?



UPDATE 11th MAY: It turns out the Times feature we refer to was premature. Portico Quartet have posted today on Twitter

"We'll be making an announcement this week. Just putting everything together with a new member."

Hang on. A feature about The Portico Quartet in the Times on Thursday by John Bungey (not available online other than via the Times paywall, not mentioned on the band's website or blog) has a picture of Messrs Wyllie, Bellamy and Fitzpatrick (but not Mulvey-above) entitled "Quartet of three thinkers," and captioned thus:

"Portico trio."

It explains that hang player Mick Mulvey has left the band after a final appearance yesterday for the first half of their gig at the Cheltenham Jazz Festival. "He's leaving to become a singer-songwriter."

The article concludes by raising the question of whether the new band with less hang and more electronics will continue to attract success.

Bungey writes: "As the band move from sunny musical climes to somewhere darker and stranger, they are about to find out."

The question is intended purely philosophically, of course. But how many players

-are there
-should there be
-do there need to be

in a quartet?