3,4. 1&2 will never fill.
Correct but only if the pipes can all carry the amount of water coming from the tap3,4. 1&2 will never fill.
The flow rate and amount of water that the pipes can handle are irrelevant,
As previously said 1 and 2 will never fill up.
3 and 4 will fill at the same moment,
No water will flow into tank 4 until the water level in tank 3 is above the point where the pipe goes into tank 4
3 and 4 will overflow at the same moment.
Again that is true. But isn't there a difference between "filled" and "overflowing"
Mine isBut then again perhaps my pedanticism is out of control.