If the faucet pressure and flow is so big that it spills out without filling the bucket, the spill could reach 4. Given that the walls of that bucket are extremely cold, the water could freeze over the drain, making that the first bucket to fill up.
Because the inlet and outlet of 2 are the same size, 2 will always be filled last, if at all. Once 5 is filled, it will spill out the edges of the container rather than back flowing.