There really is no cold end on a variac IMO. It's a single winding with a tap at each end and usually one about about 80% of the way from one of those to the other. The fourth terminal is attached to a wiper that runs the entire length of the coil. Apply power at the two end terminals and pick off power between terminal 1 and the wiper terminal. Usually terminal 1 connected to neutral so it could be considered cold but since variacs almost almost always have a single winding, everything beyond terminal one has voltage on it. In this configuration the output will vary between 0 and 100% of what ever the input voltage is. But you can also connect the line side to terminal 2 instead and that will boast the output to between 0 and (input/80%) volts or about 130 VAC in the US (with 115 VAC input).
This one is the same except it has multiple taps. However connect neutral to terminal 2 and line voltage to terminal 4 and then take the output from terminal 2 and the wiper (terminal 3) and you will be able to vary the output from 0 to 100% of the input. Note this variac is made for up to 270 volts across the entire coil but you can certainly run it at a lower, safer voltage. If you want to experiment with it. Run it off of a isolation transformer with a lower voltage AC output such as 12 VAC. The voltages involved will be a lot safer to work with. This one has other taps but I wouldn't bother with them until you understand variacs better.
The biggest fault with a variac is that it is a single winding and basically, everything is connected to the AC line. Never forget that! If your neutral terminal or the winding were to suddenly open you could have full line voltage at the output regardless of the wiper setting. That's why I suggest powering it from an isolation transformer while tinkering with it.
PS ignore the numbers of the dial. Those only agree with the output when it's wired in one specific manner and with a specific voltage input. I much prefer to to think of the output in terms of percentage of the input voltage and as a function of the percentage of wiper rotation. But even that can be altered by connecting it differently. Variacs are amazingly versatile if you understand them!