If I'm playing in a Prerelease with 4 Prize Cards, what happens if I have Beast Bringer attached to an Ultra Beast and I Knock Out a GX Pokemon as my first Knock Out?
Nothing happens. The card specifies that you must have exactly 6 Prize cards remaining but you only have 4.
Source: Unbroken Bonds FAQ (2019-04-19), TPCi Rules Team (2019-05-02)

Relevant cards

Note: All applicable cards may not be listed below.
SM10_164
SM10_164
SM10_229
SM10_229