I don't watch Futurama, so I may be wrong in how the story goes. But I think he thought that he's rich but inflation shouldn't be much lower than interest (irl, it's actually way higher), so the 4 billion is likely worth 4 bucks or at most 4 hundred.
It seems like the episode treats him as legitimately rich, but you're right: assuming targeted 2% inflation, it's like $10. That's being pretty generous too, 1000 years is a long time to go without some kind of societal shakeup event where people stop honoring 800 year old numbers on a screen as legitimately valuable. It's not like there is any entity today who would feel obligated to give you $4 billion dollars for your 11th century banknote.
