Actually, I’m going to expand on my answer, since it’s not really as clear-cut as some people might at first suppose.
In your example, it was clearly plural, but in real life, it could often go either way:
A: How much milk did you drink?
B: About four liters.
A: Four liters? Four liters is a lot of milk!
In this case, we would usually use a notional singular, since we’re talking about the milk, not the “liters”. In your first example, there are two actual, physical bottles, but in my example, liters are just a measurement for the quantity of milk. The liters don’t necessarily exist — they’re just an abstract concept to quantify the milk. Perhaps it was a single one-gallon bottle of milk that B drank :) It’s the same idea with money. We say: “Ten trillion dollars is a lot of money.” Note that there are not necessarily any actual, physical dollar bills — it’s just a figure on an account statement somewhere.