Hi summoner !
As you explained, the darkins may be from the myth of the Horsemen. Aatrox would symbolize War, Varus would be Conquest and Rhaast (Kayn) would be Death (also known as Pestilence, but Death fits Kayn better). I really like this theory because even the champions' colours respect the horses' colours: Red for War, white for Conquest and pale for Death.
As you mentioned, we could have a 4rth darkin empowering Famine, but we would have a problem with the 5th (even if Jax is sometimes considered a darkin, for some obscure reasons ... I don't believe he's one, but it is true that there is a Darkin's shape on his artwork coming from the lantern if you look well, but it seems to be Aatrox ... this is another subject). Here is the link for Jax's image with a darkin's shadow:
https://gfycat.com/fr/oblongtighteidolonhelvum
As people mentioned earlier,we could think of a whole team composed of Darkins, and I actually had this idea too ^^' I already created an AD darkin mdilaner, playing around being Famine (and killing people by starvation). First, I wanted to make a support that would eat people but ... well ... there is already one with big Tahm Kench !
And for the 5th darkin, I already read that Warmog itself was a darkin, but I actually disagree, since we have no clue about it, but that the armor is a living armor. I also read that Nautilus could be some kind of Darkin, because he was possessed by something evil, but I don't think it was a darkin, it was probably Balck Mist.
Some far-fetched theories talk about twitch being Pestilence (but he is a rat -_-), or Kindred being the 5th darkin, because Lamb would be referring to the Lamb of God. Moreover, Kindred symbolizes the end of life, it is an etheric being, which could correspond to the 5th darkin, the master of Death and empowered by the 666, the Devil. I like the idea, but I really don't believe Kindred is the 5th darkin: she is not as evil as the other darkins, and seems to bring death to those deserving it, not killing mercilessly as many people as possible. Another theory would be that Death Dance would be a Darkin, since it looks kinda like Kayn's scythe, but it is too far-fetched for me.
That is all for me, from what I read obout this subject :) I hope it pleases you guys !
Good luck in the Fields of Justice !