D.H.
The boys are doing this because they know you will come running. Sounds like they share a room. If so, the 4yo is the culprit. He screams for you and wakes the babby. The baby probably will sleep through the nite if he wasn't waken by the 4yo. Now this is setting a pattern for the baby.
I think you need to have a long "stern" talk with the older one. Explain that his behavior is unacceptable. Why does he need a drink in the middle of the nite anyway? If he can't reach for is own cup with out disturbing the ENTIRE family, then the cub will be removed.
I can't even imagine how frustrated you are because I was annoyed just reading what you're going thru.
I hope you're able to come to a resolution.