I think the 5th horseman should be called "Depression".
The four horseman were first mentioned in the book of Revelations. Back then, the economy didn't play as large a role to a country's well being as it does today. "Depression" could be seen as a modern issue, and that would be explain why the horseman wasn't named beforehand. The "Great Depression" is a good example of this horseman's destructive powers. As our reliance on the economy and money increases over time, so does his strength. He would make a formidable horseman.
(On another note, interestingly enough, Conquest is sometimes used as a replacement for Pestilence. Depending on which interpretation of Revelations you're reading, the two interchange.)
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