maybe a description of the gods themselves might help
Khorne: battle, hack/slash, martial pride, berzerking followers are mainly pissed off
Slaanesh: Pleasure, melting people down into drugs, boobies. followers occaisionally pissed on
Tzeentch: sorcery, AP3 Bolters of DOOOOOOOOOOM!!!!! nifty headdresses, plotting, scheming, followers have AP3 bolters of DOOOOOOOOM!
and last but not least
Papa Nurgle: peace, love, and crabs. happy pestilence, followers just want to give you a hug (and thereby turn you into goo but it's the thought that counts)
gameplay wise Khorne is hack n slash
slaanesh is fairly balanced, able to hack and shoot both.
tzeentch is SLOW with AP3 bolters of DOOOOOOOOOOOOM!
Nurgle is... um... tough, kinda slow
honestly though undivided is the way to go if one of the gods doesn't grab you.