"Four liters of water."
Translation:Τέσσερα λίτρα νερό.
"This is an example of, let's say, "further explanation" between the quantity and the material/type of what this quantity comprises. The accusative is used instead of the genitive.
Quantity=A PAIR (of whatever that is)=TWO (of whatever that is)
Material/Type of the quantity=SHOES
Two more examples:
He showed up with a piece of pie.=(Αυτός) εμφανίστηκε με ένα κομμάτι πίτα.
Quantity=A PIECE=Ένα κομμάτι (of whatever that is)
Material/Type of the quantity=PIE=Πίτα
There are other cases of "further explanation", extra to the "Quantity-Material" one, but let's stick to this one in this post, since there was a specific question."