WoC 2500 four horsemen of the apocalypse

Seems like a fun theme for a list! Here's my input on your choice of units:
Your non-general characters could use some defensive items. The Dawnstone is pretty great for Chaos due to how easy it is to get them to a +1 armour save.
The lack of a level 4 wizard might also become somewhat of a problem, to that you might want to consider a different lore on your wizard. I for one love the lore of Nurgle on a mounted sorcerer since that allows the signature spell to truly shine. I've also heard that the Lore of Slaanesh is really strong on a fast spellcaster. If you want a Tzeentchian wizard I'd consider giving him a disc and the lore of metal so that he can fly around shooting at things.
You should probably give that unit of Slaaneshi marauder horsemen shields for added defense and spears for added offense.
Their khornate counterpart could use a musician.

Mark of Nurgle is really strong on Chariots but your choice of Slaanesh also has merit if you want them flexible enough to avoid unwise combats.
As for the hellstriders I'd either take one more or one less so as to have a unit of either 10 or 12 when the character is added to it.
I personally wouldn't go over 3 Skullcrushers if you're going to add a character to the unit and the Banner of Rage seems kinda superfluous on them considering that they are very much likely to chew through most things that cross their path. It's also worth mentioning the risk of taking a standard bearer in the unit. While it is unlikely that they would lose a combat if they do and flee the model with the banner would be automatically destroyed which is a considerable blow to the unit if it manages to rally. For the points you'd save I'd consider adding atleast one Gorebeast Chariot of Nurgle; they hit hard and are tough as hell.

Unless I'm mistaken they only lose the Fast Cav rule if you give them light armour. Even as a designated harassment unit spears could prove useful since going from S3 to S4 on the charge would make them a potent threat for the opponent's chaff and artillery which means that they can't be ignored as easily which equates to attention away from the rest of the army.

Ah my apologies, I knew it was one or the other. I backed the wrong horse 😂