GSTR 4 (annual)

CMP-08 filing must be done by all composition taxpayers (including service providers opting for composition scheme under Central Tax (Rate) Notification number 2/2019 dated 7 March 2019).
The composition dealers have to pay tax in CMP-08 challan-cum-statement every quarter by the 18th of the month following the quarter.

The composition dealers have to pay tax in CMP-08 challan-cum-statement every quarter by the 18th of the month following the quarter and requires to file return annually in GSTR 4 by 30th April of following financial year.

Thanks sir

But the last date to file GSTR 4 annual return for the financial year 2019-20 is extended up to 31/08/2020

Yes, The due date of the form was lately extended to the 31st August 2020 from the 31st of July 2020 as the form was not available on the GST Portal. 
Now, Form GSTR-4 for FY 2019-2020 has also been made available on the GST Portal.

is offline tool available or is it necessary to enter each and every purchase details..any solution to it

Update : 06th August, 2020

Offline utility to prepare Form GSTR-4, Annual Return by the composition taxpayers is now available in download section of portal. This excel based utility is designed to help taxpayers to prepare Form GSTR-4 annual return offline.