“Trading 4s is a jazz term used to describe when two soloists play alternating four-bar phrases, as though they were having a lively conversation with chords.
“In North Coast Brewing’s tribute beer to Jazz legend Thelonious Monk the brewery trades 4s not with musical instruments but with yeast and malt. The result is a playful duet in the chord of B (B standing of course for Belgium).” Read the whole review on the BeerSweden website.