There are only two options left to rectify the mistakes.
1. Add the previous year libility in your next financial year gstr4 return.
2. while filing cmp 08 add your tax liability which you have forgot to add in table no. 6 of gstr4 and set off from negative liability adjustment. in both cases you need to show the departmental officer that you have rectify the mistake by making changes in case of any enquiry.