In effect, Tzeentch is the drive of the actions of the four, and has every intention of betraying them to possibly leave the immaterium in a state that compliments what it is, which is pretty much a Great Old one. I should also note that the betrayal of the Iron-men is most likely the resut of Tzeentch easiyl corrupting them, for whilst they were sentient, they had no souls, and were as easy to mould as clay for it's efforts, yet Tzeentch has a dedicated once-human Greater Daemon for the purpose of corrupting machines alone. It's probable that Tzeentch wanted the constructs for it's own use, AKA the possible construction of a vessel to possess. Slaanesh for example, has a purposed vessel to ride about in to fuck Fulgrim, Tzeentch literally is on a time limit before Nurgle wins it out.
In fact, Tzeentch's individuality is also shown with it's actions when it broke it's staff, and when it threw it's Lorfs of change into that one well because it was UNWILLING to sacrifice itself for the power that Kairos later received through going through it, Tzeentch literally has shown it's own self-serfving nature.
Oh and lastly, why do you think Lorgar is all kept pent up in a palace, 'Meditating on the true nature of Chaos' An impossible feat essentially
after the Horus Heresy ended? Because Lorgar is the key to Tzeentch's master plan being unraveled, Lorgar, gaining any knowledge of the now defunct 5th Chaos god is now secured by the Daemon Primarchs now-blind faith, and is none the wiser, living in effectively, self-imposed house arrest.
Hell, it's also the explanation about why still-living cultists to chaos and those enslaved and force to worship through torture, via mass prayer and whipping are present, there is an ever constant need for the ruinous powers to expand for them to do anything, in order to gain opportunity and actual progress over what Malal would have undone, had the Heresy granted it any sort of spotlight.