My understanding was that the Chaos Gods very rarely shared their toys. Be'lakor and Horus being the two most notable representatives of Chaos Undivided.
Most of the primarchs are fairly straight forward:
I don't think Horus was ever elevated (he was simply invested with power) but which god elevated Lorgar?
You are making a misinterpretation here that chaos undivided is some form of committee that each of the gods sit on and they vote. There are untold numbers of beings in the warp who worship and follow chaos undivided.
Lorgar is also not the only primarch to follow the path Pertuarbo is also a prince of chaos undivided.
As defined by games workshop in the Codex: Chaos Space Marines (3rd Edition, 2nd Codex), paraphrased below.
Chaos undivided is considered, by its worshipers, to be the true essence of chaos. They see each of the chaos gods as one part of the whole, or possibly worship them all equally. There are plenty of demon princes that follow the path of chaos undivided.
The very essence of chaos undivided itself can raise an individual up it can be considered to have a will al of its own.
The 4, in vying for supremacy, will attempt to sway or convince that there is no such true being and only they can offer the true gifts of the warp but this is proven to be untrue by the many beings who have been raised up to the status of demon prince of chaos undivided.
So in answer to your question the force that is Chaos Undivided raised up Lorgar on its own.
