WHICH TANK FILLS FIRST? ft. Simone Giertz

I look at this from a different perspective, real world!
The 1 or A tank fills first on both the main riddle, and the more complex one shown at the end.
I have actually worked with such things ranging from water treatment and industrial processes to doing the grade setting while building cascading wastewater ponds.

So in the real world, "Full" is defined as the maximum level the tank is designed reach, which is often defined by the presence of an overflow port.

In the replicated blueprint, Tank 1 is the surge tank, and is only designed to fill up to the level of the pipe to Tank 2. It is designed this way to keep sand and such out of the system, and it is so large in case there is a abnormal surge of liquid, such as when the bench collapses, it can overfill some and then meter it into the system. But it is defined as being full if it is at the level of the outflow pipe.

Now Tank 2 is just for aeration, gives a good splash and churn. 

Tank 3 is for separating clear water from floating debris, so it has the outflow port near the bottom of the water column so that the floating debris doesn't get transferred to 4.

Tank 4 is the settling tank, and not shown in the blueprint is the distribution pump that draws out of the top of tank 4, controlled by a level sensor. 

So, on initial startup Tank 1 fills up first, and begins flowing out of the overflow into Tank 2. 

Once Tank 3 reaches the level of the the inlet on Tank 4, Tank 4 fills to the same level as Tank 3, at which point they begin filling together, along with Tank 2, until they reach the level sensor and turn on the distribution pump. 

So in my scenario, "Full" is whatever the designer planned it to be, and not all tanks are designed purely for holding liquid, they often are to contain liquid while a process is occurring, and so you have to look at the design of the system to determine what the proper tank level is.