In Greek the genitive case του νερού means of the water and not of water. The genitive case is most commonly used to show possession, but it can also show a thing's source or a characteristic/trait of something.
If you ask for two glasses of water, I'll give you δύο ποτήρια νερό. However, if you ask for two water glasses, then I'll give you δύο ποτήρια νερού.
Same for one bottle of water in greek is ένα μπουκάλι νερό , but one water bottle translates as ένα μπουκάλι νερού.
If we were to translate τέσσερα λίτρα νερού then this would have to refer/characterise something that would indicate "volume of four litres of water". Hence it could be like [a water bottle with the capacity of] τέσσερα λίτρα νερού.
Hope this helps!
Yes, "ένα κιλό κρασί" is informal, of course I agree. But nevertheless it is commonplace in Greece. That was the reason for my question above. Duolingo users want to learn the language, perhaps for visiting the country and for every day use. So: What's right or wrong...?
Language is alive and is evolving; language rules exists for this reason, to distinguish right from wrong and accuracy is an important part of good communication. So if some people use “kilo” instead of “litre” it’s just wrong, then you choose of course which one to use.