With effect 01st April 2021 CBIC notifies that a dealer registered under GST has to mention 4 digit of Harmonised System of Nomenclature Code (HSN Code) if his Aggregate Turnover in the preceding Financial Year is up to rupees five crores and he has to mention 6 Digit of HSN Code if Turnover in the preceding Financial Year is more than rupees five crores.
Source Taxguru
See also Changes in GST Rules related to mentioning of HSN code
