The correct answer is Dog 4.
Here is why the other paths fail when you look closely at the image:
Dog 1: The channel leading to Dog 1 is completely blocked right at the wall of the first left-hand square container. No milk can enter this pipe.
Dog 2: While milk can enter this vertical pipe from the container, the pipe is completely sealed and closed off at the very bottom.
Dog 3: On the right-hand side, the small downward pipe positioned directly above Dog 3 is entirely blocked right at its exit point from the square container.
Dog 4: This is the only path where the channel is entirely open and unobstructed from the main pour all the way down to the pipe's exit point.
Therefore, Dog 4 is the only one that will actually get to drink the milk!