The Clue Chaturthi : Quartet :: Ashtami : ___
The Answer OCTET
Chaturthi means the fourth day of the lunar month, representing a group of four, or a quartet. Similarly, Ashtami refers to the eighth day of the lunar month, which corresponds to a group of eight, or an octet. Just as a quartet has four members, an octet has eight. Therefore, the answer "octet" fits perfectly with the relationship in the clue.