Possible Clues:

• Group of four fish, fourth in a series
• Tips for trombone joining a kind of jazz quartet
• Gift of calling birds, say
• Empty tune by kind of jazz quartet
• The classical elements, e.g
• Looking back, transportation system for alien becomes four of a kind
• Set of four
• The less hot kind of jazz quartet
• Brahms' symphonies, e.g
• Group or set of four