Right, this might sound strange but just hear (read) this out.

So, as I se it the chaos gods havn't been around for all eternity. Not since he beggining of time anyway. As I understand the necron codex (been a while since I read it though) the necrontyr where being swamped by the old ones and found the C'tans.... You all know that. However, When the old ones started creating races to help fight the necron(tyr) things didn't go so well. As when they made the orks, they prooved to be to "unreliable" (I think there's a thread about that somewhere). The Eldar turned out to be usefull but here's where things starts to become weird. Didn't the eldar somehow make daemons appear in real space (do to their close bond with chaos), and wheren't these daemons like.... Just unbound dudes with no masters? In other words, these daemons where not belonging to one of the fout majot powers.

Further along, Slanesh (I can't spell it), where supposed to be a entity as soon as the eldars came around right? And later, during the fall, Slanesh became a true chaos god. Now Slaanesh was based on Eldar emotions. Aren't the other 3 gods based on human emotions?

As I se it, the three other chaos gods only awakened due to the minds of human. In the warp, the more beings that belive in you the stronger you get. I'm not sure where I'm going with this but as I se it, the chaos gods where not borned untill the human race started thinking and therfore where not arounds untill quite sometime after the whole "war in the heavens" (Necrons Vs the old ones) was over. Note that the humans appeared before the fall of the eldars.

This might not be the case but I don't think that I'm completely wrong.