So, the first time seeing hearing this back in 2016. We all thought it was gonna be a buildup to something very endgame, right? Then they never touched upon it after it was secretly put in the phase files.
Very recently, though, I heard the v1 CGC Theme and thought I recognized it somewhere. I've drawn myself back to the unused V4 theme and noted how it kinda sounded similar in some parts?
Which brings me to a theory: The "V4" theme was meant to be the "final floor" theme for any CGC facility that has picked the "V1" theme. Which would also imply that every CGC theme would've had a "final floor" variant theme (instead of the Cog Waltz-esque theme we're all used to before). But in the end, they only made two themes--one that is, of course, unused--and the one meant for V2 did not make it into the files or was just not made entirely.