There are 3+3=6 empty bottles left after drinking.

Replace 6 empty bottles with 1 bottle of water.

Drink the rest 1+(6-4)=3 empty bottles.

Unfortunately, one is missing.

There is no water.

A * * * drank 3+ 1=4 bottles of water.

If you are really thirsty,

Pick one up on the way,

Drink it.

Hehe ~ ~