So there are five Darkin, but...
Three of them currently follow themes of The Hour Horsemen:
The Horseman of War uses a sword, which Aatrox also uses. This is befitting of his current ingame character. He inspires war, and inspires those losing to fight, or be forgotten.
The Horseman of Conquest uses a bow, which Varus also uses. This one is interesting because as I recall, the Horsemen is also referred to as Pestilence, which sort of fits Varus through the seeping corruption on his body, and his ingame visuals for spewing blight.
The Horseman of Death resides in Kayn's hands. Pretty self explanatory. He's an assassin and kills people. It could also be assumed that his own death to Rhaast ties in with this theme, should be lose control of the weapon.
The last Horseman is absent. I'd imagine it's going to be something to do with Famine. Famine's 'weapon' of choice was a scale, and as the name implies often involved starving out populations as a means of culling them. Maybe we'd even see a female Darkin pop out of this?
The fifth Darkin is unaccounted for, as there are only four horsemen. However, I figured I'd make a short little writeup to draw parallels between these characters.