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In the pics on that page, you can see that the four-pin connector is for the cable.
Keyboards with an AT/XT switch can switch between the XT and AT protocols, but yours is missing the switch: I guess it talks AT only, because that is the newer one.
AT and XT used the same large 5-pin DIN plug. PS/2 is the same interface as AT but with a 6-pin mini-DIN plug and another pinout. You can find that pinout on Wikipedia and on many other sites.
The pins are for +5V, GND, Clock and Data. You can often figure out which pins are GND and +5V by them being connected to many components on the PCB whereas the Clock and Data pins should go to the controller chip.
Luckily, this is a one-sided PCB so you can see all traces. There are pictures of cable there on Deskthority's wiki and a black wires is usually a GND wire.
I'd suggest that you get a PS/2 - to - USB adaptor: and active one, ie. one that has electronics inside. (also called "protocol converter"). That's because PS/2 isn't hot-swappable, but the adaptor is with its USB plug.
If you connect Clock and Data swapped to an active PS/2-to-USB adaptor, that should usually not break anything: a LED might light up but it will just not work.
With the USB adaptor it should work with any computer that has USB ... except Mac, because there are no Command keys. If it had had Windows keys, then those would work as Command keys.
