The transformer is made for systems that provide a constant voltage level. The distribution system is much like a power distribution system. You have power in your house. You add a workshop and simply parallel it onto the system. A neighbor builds a new house and simple parallels on. The neighbor builds a work shop and adds it on. No change is required to be made with the existing system when additions are made.
If the amp fed the speakers directly you would have to series and parallel to match the impedance, this could get messy quickly. It also allows you to set the power level at each speaker. High power in a noisy area and lower power in a quite zone.
The speaker is connected to the 8 ohm taps and the amp is connected to the wattage taps.
We start with this diagram.
These relationships give us what we need to solve the riddle?
As we go up the primary we have a higher turns ratio and less power being supplied to the speaker. If the line is 70 volts how much current would be applied the primary to produce 4 watts power? (P=E*I) When you find I you can calculate Z...........
P=I^2*R so what current produces 4 watts power in an 8 ohm load?
if all is well you should be able to determine the Z and turns ratio for each tap. Then double check your work using the ratios given above.