The techniques is called '100V line' in Europe, I think the USA might use a slightly different standard?. Essentially it's like electricity distribution - use a high voltage and a low current to prevent loss over long wires.
So at your 4 ohm amplifier you have a 100V line transformer, that converts the 4 ohm impedance to 100V line (the impedance doesn't matter, that's the whole point of it). Then at each remote speaker you have a 100V line transformer (except a smaller one) to convert it back down again, the transformer will normally have multiple taps, so you can adjust the power output to each speaker individually - the taps are labelled in watts. As already suggested, instead of using the taps, you can use a large wattage wirewound potentiometer between the transformer and speaker to give fully variable levels.
Calculations are simple - again, that's the whole point!. Just add the wattages of the transformers together, so if your main amp is 100W, you need a 100W to 100V line transformer to feed the line. You can then have a maximum total of 100W of speaker transformers - so perhaps ten 10W ones. If you wanted to connect twelve 10W transformers you would need a 120W amplifer and transformer.