Mysterious Cryptogram

You received a new cryptogram from Graham. The cryptogram is [ 6 16 25 9 / 10 13 22 9 / 19 18 / 24 25 22 24 16 9 / 13 23 16 5 18 8 / 24 12 13 22 8 / 10 16 19 19 22 ] [Key : 4]

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