I think anymore than 3 cups and you'll still have caffeine in your blood system at night, which doesn't seem healthy. I think it's easy to fall into a pattern of increased caffeine use through the circular pattern of drinking too much and getting terrible sleep, then ramping up the dosage the next day in order to feel awake.

I think it's important for people to recognize this little quirk and not to spiral into a pit of its tasty dark magic.