Coke from prewar might be one of the only safe things to drink, so counterfeiting it might be extremely valuable. People are willing to pay a lot to be sure they won't be infected with a plague or poisoned.
Alternately, people could just have an association to safety with it, the problems with water supply being mostly solved by the time in question.
"Coke is safe to drink." is a great meme for sales even if the danger of not drinking it is really low.
For people to spend loads of resources on a beverage means one of only two things:
- The beverage is required for their continued survival.
- The person in question has an abundance of resources.
All rational answers are going to need one or both of these.
Since Coke isn't a particularly efficient approach to providing it's nutritional benefits compared to, say, its constituent parts, there needs to be a reason it's desired or that it's needed. Warm fuzzy feelings or a severe lack of water could suffice. As Flox points out, status is another possible reason.
Caffeine and sugar are useful in various circumstances, some of which are medical, also.
(Sidenote: Without something to correlate the number, any price tag is mostly meaningless. 4k in dollar bills requires we know what inflation's been doing. If you mean it's equivalent to $4k of modern money buying munitions, say, we have a much more concrete image of the value of this beverage.)