Should I be concerned or is it a minor issue?

I am thinking of buying a rural property but there are discrepencies as follows.

The Registry states 20,000m2 ownership; but the Catastro states 24,000m2 plot.
No physical boundary exists around the land, so I'm concerned that someone could make a claim on 4000m2. the current owner has had the property since the early 90s.
All the plans I've seen always refer to the 24,000m2 plot, so I think a mistake was made when the curent owner bought the land. Interestingly, his passport number and not NIE is on the escritura!!

Ideas anyone ?