I do not believe RQ is a greater god any more or less than any of the gods in 4e. As far as I can tell (it has been a while since I was deep in 4e lore - which I love by the way), all of the gods given stats in 4e are just "gods" in such that they don't really ever provide a distinction. They definitely
are not divided into such groups in the PHB or DMG, but I can't be sure it wasn't listed somewhere else, but I don't think so. I will say I did an extensive search on this subject (back when I played 4e) and determined there was not "official" evidence of lesser or greater gods. If you have evidence otherwise, please pass it on as I looked for it for a long time.
The only distinction I could find was between gods and exarchs. There are several gods in other editions that are listed as exarchs in 4e.
It is also worth noting that per the DMG, and mentioned elsewhere as well, primordials killed many gods in the Dawn War. And none of the primordials we have stats for get above lvl 35.
DMG pg162
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Regarding the RQ and Orcus in Prince of Undeath. It is not an avatar, she is the real deal. From the adventure's conclusion:
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It is important to note the for the final battle Orcus is "empowered," but this only takes him to lvl 34. Therefore, there is as much, if not more, proof to suggest that RQ is canonically among the weakest of the gods in 4e, around lvl 34 or maybe 35.
UPDATE: I knew I remembered reading something about demigods and lesser deities. I found it in the Manual of the Planes. In the section on Avandar it mentions "demigods and exarchs" as entities that attend the two "greater powers" (Corellon & Sehanine) and in the section on Celestia it mentions Moradin is the first among equals (between Moradin, Kord, & Bahamut) and that he governs over a court of "lesser deities and powerful exarchs." Then in the section on Hestavar it mentions: "Three greater deities (Pelor, Erathis, & Ioun) make their homes..."
So there is mild evidence of a distinction, but IMO in 4e the terms "greater powers" and "greater deities" are synonymous with the term "god" and the terms "lesser deities," "demigods," & "exarchs" are basically synonymous with each other. We are definitely not given a clear picture of their differences (between exarchs, demigods, and lesser deities) and they have a wide range of power levels. Also, it is worth noting that if Moradin is an "equal" and a "greater" god than by association you can pretty much put all of the gods with stats in 4e in the "greater" god category. Personally, I don't think that is a significant distinction in 4e.
PS In the official "Scales of War" AP for 4e, Tiamat killed Bahamut (but the PCs can bring him back). So being one level below your foe doesn't mean you can't defeat them, and gave us this pretty epic image:
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