https://upload.wikimedia.org/wikipedia/commons/thumb/e/e1/Apocalypse_vasnetsov.jpg/1200px-Apocalypse_vasnetsov.jpg
1 
Darkin Blade
2
Darkin Arrow
3
Darkin Scyth
4(
+
) Darkin Scales
5
Darkin Crown... Halo? Circle... Chal...Chillicar? BOOMERANG BLADE!
But for real it's Jesus's crown of thorns. Sivir is Jesus. She died... came back to life, etc.
First, it's been confirmed that the five remaining Darkin are not meant to parallel the Four Horsemen of the Apocalypse, no matter how close the resemblance may be right now. Sure, Aatrox, Varus, and Rhaast seem to parallel - at least in arms - War, Conquest, and Death, but the similarities end there.
And, both to address one of the last two Darkin being Famine and your idea of "the Darkin Scales", here's a bit from the Great Darkin War details on the Universe page:
To slay legends
Ultimately, they were defeated by the mortals they sought to enslave. With magic and cunning, the greatest of the darkin were forged into living weapons, and used against those that remained—but anyone who takes up such a weapon will become little more than a host, a vessel for the immortal sentience within it.
"Living weapons". While the Blade of Millennia isn't a weapon of war (as it's a reaping scythe, not a war-scythe), it remains a weapon. So, we have a greatsword, a bow, and a scythe.
Scales, however, are not weapons. They're a tool of measurement.
As for Sivir and the Chalicar, if the Chalicar were one of the Darkin weapons, it would have already possessed Sivir by now. It isn't a prison for any Darkin souls - it was just used as a tool to invoke the light of the Moon to burn away the Darkin's immortality. Chalicar =/= Darkin weapon, Sivir =/= 5th Darkin.
Aatrox, Rhaast, Varus are confirmed to be around.
Naganeka, Valeeva and Cebotarus fates are unknown.
Kayle and Morgana might be Darkin, might just be their own form of Ascended, might be Baccai.
"Let me show you your error, Kayle! You are not worthy of ascension! You, are not worthy of the worms!" "Die, Kayle, knowing you potential will never be reached." "You would bind yourself to this realm. Morgana? Fine, let me show you the ground!" "You disgust me, Morgana. Die in the filth of your bound flesh!"
“Idri-Mi,” she
said, handing off Ta’anari’s heart, “take this and the Chalicar to your weaponsmiths. We will need to take a different approach in dealing with the rest of the...”
Myisha paused.
“Wait, what was that old word?”
She snapped her fingers.
“Ah, yes. That’s it. Darkin.”
There said that there are 5 Darkin that Runeterrans know of, doesnt mean that there are only 5.
