How to correct the area mentioned in land register document at tamilnadu.

You first send a legal notice to the vendor to his last known Indian address, get it returned undelivered after that you submit a rectification deed to the concerned Sub registrar office, if that is rejected then you approach court with a suit for declaration of correct extent of property and mandatory injunction seeking direction to sub registrar concerned to permit you to execute a rectification deed 

Who is in possession and occupation of the property?

The land admeasuring 2400 sq ft is stated to have been purchased, wherein the earlier transaction was rectified from 8000 sq ft to 2400 sq ft in 1998 document, was it not noticed by your father at the time of finalisation of the transaction / availability of 2400 sq ft. Please appreciate the fact  that there is vast gap between the piece of  land admeasuring 8000 and 2400 sq ft, which cannot be acceptable by vendor (seller).

Inspect the sub-registrar records and find out whether subject property is 2400 sq ft or 8000 sq ft and transfer of its title for the area mentioned therein he records. If it is 8000 sq ft in records and your father had transacted for 2400 sq ft out of 8000 sq ft there is no issue to be resolved / proeeded.

However, if the area is 2400 sq ft issue notice to vendor at his last known address besides Sub-registrar followed by a suit for declaration as advised by learned expert Mr. T Kalaielvan, which I agree.