Due Date to file FORM GSTR-4 is 31.08.2020. It has been extended vide Notification No. 59/2020 - Central Tax.
FORM GST CMP-08 and FORM GSTR-4 are different Return.
The due date to file Challan cum Statement ( CMP-08 ) for the Q1 of FY 21 is 18th July 2020.
@ Sanjeev Agarwal Sir, you will not find any notification in the CBIC website regarding due date of CMP-08 being 31/08/2020 because no such notification was issued by the Department till now.