Hi fellow miniwargamers!
This is a short guide that describes the four Chaos Marks and what they do, and when they are usefull. I hope this tactica helps you decide which marks your army will include. Note: This applies for units that you can give an icon of one of the four patron gods.

Tzeench- This mark, happens to be my favorite. It gives you a 5+ invulnerable save, or improves the one you already have. This is a huge advantage because when normally, you would have suffered instant death, now you have a chance to save your squad from destruction. This works great against the strongest of enemy fire, such as the destructive Big Mecs Shock Attack Gun * I might of spelled that wrong.
It works especially good in anahilate so that it makes it harder for you opponent to get a kill point. Basically it gives you a chance to save your squads from instant death.

Khorne - Khorne's mark enhances you close combat skills by giving you an extra attack. This mark is most usefull against armies that prefer close combat such as Tyranids or maybe even Orks. Use this mark if you want to have a close combat army that is good at winning assults. In battles that you know your oppenent will try to assault you as much as possible, this mark is best.

Nurgle- Nurgle's mark is also very effective, but it works best against armies that like to shoot you. His mark gives you +1 toughness from range. That makes it harder for the toughest guns to wound you, but doesn't stop every weapon. Use this mark against ranged armies such as the Tau.

Slaanesh- Slaaenesh gives you +1 initiative, which is useful in close comabt. That extra initiative could allow you to strike first, even against units with higher initiative, such as the Eldar. This mark will help you defeat your opponent before they can strike you and might save you squad from destruction.

Of course, if you made an apocalypse army, why not include every mark. All four of then are usefull, but it may also be good to have 1 mark that you army is centered on. That will help you specialize your army or if you include all of them, it will give you a wide range of skills.

Thanks for reading this, I hope it has helped you in some way.