Who or what speaks on behalf of Chaos Undivided?

When a chaos space marine legion is dedicated to chaos undivided and communes with them... who or what do they communicate with? Is there some sort of neutral party in the warp who handles negotiations between the four, or is there a representative from each, or do they commune with whatever answers unbiased i.e. Nurgle might answer one day, Khorne the next etc.

Single god legions are fairly straight forward but due to the disdain each chaos god has for one of the others, it must make chaos undivided a tricky thing to deal with in a practical sense.