What I have just done is:

• Downloaded a trial version of Poser Debut from the SmithMicro website; because for Poser Pro 2012 and Poser 9 I couldn't find trial versions
• In the Poser Debut content dir, there IS a dir 'RuntimeGeometrieshorseHi'; it contains the file 'horseHiP3.obz'; which I have unzipped to the file 'horseHiP3.obj'
• Loaded the legacy DAZ Full Horse Carousel into DAZ Studio
• When asked for the file 'horseHiP3.obj', located it in the dir above

And voila: Eventually the legacy DAZ Horse Carousel HAS the correct P4 Horse geometry to make it seemingly function correctly ! Hallelujah !