Four keys are old (4)
I believe the answer is:
aged
This clue could be a double definition.
'four keys' is the first definition.
'aged' can be an answer for 'keys' (A, G, E and D are musical keys). I am unsure of the 'four' bit.
'old' is the second definition.
(similar in meaning)
'are' acts as a link.
Can you help me to learn more?
(Other definitions for aged that I've seen before include "not like a callow youth" , "Got older" , "Like Uncle Arly" , "Old or ancient" , "Geriatric" .)