Situation is like, user deleted from the system and his shared OneDrive file also deleted. How can I get back deleted file?
You can recover/restore files from user's OneDrive recycle bin. Deleted files are stored in the bin for 93 days.
More information is available here
user needs to go to the website version:
once loaded on the left hand side menu at the bottom click on "Recycle Bin" -> select item and restore:
4 months ago is pushing it. Depends on when the user was deleted? by the sounds of it it was provisioned a month ago which goes inline with the files missing as it takes 30 day to provision. Files might still exist.
to restore the user. Go to central admin -> under users on the left hand side select "deleted users" -> select the given user and hit restore:
finally assign password to user.
now login to one drive:
find the file and restore -> save file in SharePoint or 365 group.