Eng->Ru Exchange fours

 Что такое "exchange fours" в контексте музыки?

Несколько примеров из Гугла:
The ouds and guitars exchange fours and eights, getting kissed by a flute.
Cheek and Yahel exchange fours for a few choruses working off each other’s ideas and finally exploding into collective improvisation...
Sitting in the audience at the Festival and watching Paquito D’ Rivera spontaneously pick up his clarinet and exchange fours with one of our fabulous pianists was an extreme highlight.

Заранее спасибо

(кросс-пост в ру_транслейт)