As per instruction to GSTR 1, it will be mandatory to specify the number of digits of HSN code for goods or services that a class of registered persons shall be required to mention as may be specified in the notification issued from time to time under proviso to rule 46 of the said rules.
Vide Notification No. 78/2020 – Central Tax dated 15th October 2020, it is mandatory for the taxpayers to report a minimum 4 digit (Where the aggregate turnover of registered person in the preceding Financial Year is upto 5 Cr) or 6 digit of HSN Code (Where the aggregate turnover of registered person in the preceding Financial Year is more than 5 Cr).
As per NOTIFICATION NO. 84/2020-CENTRAL TAX [G.S.R. 700(E)/F. NO. CBEC-20/06/04/2020-GST], DATED 10-11-2020, QRMP can be opted only by registered persons having an aggregate turnover of up to five crore rupees in the preceding financial year.
Therefore, it can be construed that registered person who has opted for QRMP has to report 4 digit HSN in table 12 of GSTR 1.
Kindly refer the GSTN advisory for detailed reading (Refer : Implementation of mandatory mentioning of HSN codes in GSTR-1)