While skimming through the thread about "The LoL Villain" someone wrote about Mordekaiser and Hecarim and I came to a conclusion:
The four Horsemen of the Apocalypse symbolize four disastrous events.
These "events" in order of their appearance are:
(They are also part of Yorrick's Skillset, who in return is a gravedigger from the Shadow Island!)
The interesting thing is, that the first and the second, Conquest and War, really fit Mordekaiser and Hecarim! While not exactly Horsemen (Well, in case of Hecarim you could take that term literal) they fit the description:
Mordekaiser was the First to enter Valoran.
He is a General of some unknown force. (Conquest)
As stated in his Lore, Mordekaiser is drawn to the sick and ill. (Pestilence)
Hecarim was the Second to enter Valoran.
He wants to wage war and destroys the countryside of Demacia (War)
When Hecarim was confronted by Demacian Forces, he said:
"This is merely the beginning. No mortal army can withstand the might of the Shadow Isles."
(Hint to Famine and Death?)
This leaves me with explaining why Yorrick came to Valoran. Maybe he functions as a Herald to the dark Host?
I didn't bother searching if such a Thread already exists, so have mercy.