I usually give a quick and 'dirty' definition of the shi-yaku-jin as, "the Japanese equivalent of the four horsemen of the apocalypse".

On the surface it's more true than not. However, if you know anything about the concept: kami, it's not exactly correct from most viewpoints.

You could map shi-no-kami (kami of death) to death, ekibyô-no-kami (kami of plague) to disease, kyô-no-kami (kami of disaster, especially crop failure) to famine, and if you think about it as class warfare, binbô-no-kami (kami of poverty) to war. But there are several differences between the horsemen and the kami.

Not surprisingly, given the mythology they're embedded in, the horsemen are generally thought of as being singular and universal.

The kami on the other hand are both multiple and extremely location specific. Just as each location is different from all other locations, so the kami of each location are different.

Another difference is that the horsemen are viewed as disasters — as something "inherently evil and unnatural".

The kami are instead viewed as natural processes — certainly unpleasant to experience and to be avoided if at all possible, but also necessary to the functioning of the world, and not "evil".

And finally, the horseman are apocalyptic — a terminal ending of things.

The kami are parts of a great cycle — neither beginning or ending, but in continuous transformation.