In that case Rhaast would have likely referred to himself as bloodlust and not death.
Also the first horseman is either depicted as pestilence or conquest.
Riot could simply make a darkin for each theme.
And the idea with the 4 horsemen is just the idea for a theme for the remaining darkin,it wouldn't represent the darkin race as a whole or mean that the darkin are the actual horsemen.
I mean Nasus is based on Anubis,but that doesn't mean that Nasus and Anubis are the same characters,nor that every ascended has to be based on an egyptian god,as the egyptian gods obviously do not exist in runeterra(so for example Rammus could turn out to be just a small ascended,even though there is no egypt god in the form of armored armadillos)