I think I figured out the problem you guys are having with condensing the armies into four codexes. You need five.
>Imperium
Space Marines, SoB, IG, AdMech,
>Chaos
Space Marines, Daemons, LotD, (Dark Admech, Traitor Guard)
>Alien
Eldar, Dark Eldar, Harlequins, Dino Eldar (/Kroot), Taudar, Necrons
>Tyranids
Fukken Tyranids
>Orks
Goff, Bad Moonz, Evil Suns, Deth Skulls, Snakebites, Blood Axes
Honestly, I'm not sure Chaos really needs their own codex since they're just Imperium+Daemons, which the Imperium have now.