Four keys get old (4)
I believe the answer is:
aged
This might 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 not sure about the 'four' bit.
'old' is the second definition.
(similar in meaning)
'get' is the link.
Can you help me to learn more?
(Other definitions for aged that I've seen before include "Elderly" , "Advanced in years" , "Mature" , "Matured like wine in wood" , "Long in the tooth" .)