I got halfway through P4 Golden before putting it down for stupid reasons, but absolutely loved it. I am vaguely aware of who the killer is. Not exactly sure where I am in the story, but it's after the camping trip and before Naoto joins (I'm assuming she joins at some point).
I did not play either P4: Arena or P4: Ultimax (Is Ultimax a full sequel, plotwise, to P4 Arena, or is it just an expanded story mode?), but bought both in anticipation of beating P4:Golden. (Yeah, I know.)
I just plowed through all of the Free Dance songs in P4: Dancing All Night, and am at the point where I can't get any more achievements, two characters aren't unlocked in the store for costumes and accessories, and one is completely missing (I need to play through story for the unlock).
If I played through the P4:DAN story mode, would it ruin plot from the earlier games?
I'm considering just buttoning through all of the story beats with the visual novel skip function and keeping my eyes closed or something and just coming back to it later.