Hi all. I apologize for bumping this thread, but I've managed to finally locate the sample library that sourced the remainder of the vocals used in the opening.
They come from a library known as Midi Mark Vocal Bytes, a vocal-focused sample library from 1995. From what I understand it was a very popular library among artists back in the 90s, and even has been used in a few games including the first Grand Theft Auto, Super Monkey Ball 3D, Lethal League Blaze and even Remix 10 from Super Mario Run.
Below is a link to all the voice samples heard in Wario Land 4's opening that I could find.
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Enjoy!