Hi brneyes, I am not sure what they are saying is a credit, if you have not paid the balance yet. Sounds like a price adjustment, which is a good thing. You might should call them and see if this is still your best deal, and ask about the upgrade.
The 4A is not the fare code, it is the cabin type and location. 4 meaning an inside cabin, and A meaning the Riviera deck on the Liberty.
The fine print usually mentions 4A as the minimum type of cabin the promotion applies to. I just read the fine print for Past Guest promotion, and it looks like it is getting complicated. The fare code was the CDP1 mentioned in the last line of the fine print.
I think you are doing the right thing by doing your homework. And I have to agree that using the site that has the tabs is a great tool. I have been doing it for several years.
Congrats Master Cruiser!