Possible Clues:

• Four on the fiddle?
• Common chamber group
• Mozart's "The Hunt," for one
• Classical music group ... or what the four sets of circled letters make up?
• Cello, viola and two violins
• 3, 16 down and two 5's compose it, as have 4, 20 and others here
• Two violins, a viola and a cello
• Chamber music group
• Chamber music for four