If the pressure is high enough number 1 will fill first. If there is a steady flow slow enough containers 3 and 4 will be full at the same time. Unless container 3 and 4 are sealed at the top containers 1 and 2 will never fill and 3 and 4 will spill over. If the force of the water is strong enough number 1 will fill first and then 3 and 4 and #2 will never fill unless # 1, 3 and 4 are sealed.
Water always finds it’s level. There are no tops to any of the four tanks to create any pressure being the cause preventing the water to fill any tank completely. The maximum water level possible is the equal rim heights of tanks three and four. After the water passes through tank three reaching the upper end of the pipe connecting three and four the water will spill into four. When the water level of three and four reaches equilibrium. These two tanks should fill relatively equally until the water spill over both their rims. Both tanks one and two are higher than the level of the rims of the lower tanks so they will never fill.
Tank Number Four {4} Will be the first to fill, as the water pressure keeps filling 1,2,3, the water will need some place to go, thus the fourth tank will be the first to fill. Once the water capacity has been reached in tank number four {4}, water will have no other place to go than to tank three and when that is filled it will be tank two and finally tank number one.. The tank will fill backwards, 4,3,2,1, when tank number one is full it will over flow if the tap is not turned off.
3 and 4 simultaneously. Being at a lower height, water will fill the two tanks first.
Since they are both at the same height,
Once the water level rises above the pipe level (outlet of Tank 3), the level in the pipe and tank 3 will be the same.
Once the water level reaches the inlet of Tank 4, the water will fill tank 4 till the levels in 3 and 4 are equalized. After that, any excess water will flow so that the water levels in 3 and 4 remain equal.
Though, 3 would fill up a tiny bit earlier than 4, because it takes a tiny amount of time to reach equilibrium in 3 and 4.
https://youtu.be/BilSAM8cLtc?t=1m20s
Since they are both at the same height,
Once the water level rises above the pipe level (outlet of Tank 3), the level in the pipe and tank 3 will be the same.
Once the water level reaches the inlet of Tank 4, the water will fill tank 4 till the levels in 3 and 4 are equalized. After that, any excess water will flow so that the water levels in 3 and 4 remain equal.
Though, 3 would fill up a tiny bit earlier than 4, because it takes a tiny amount of time to reach equilibrium in 3 and 4.