Heh, Nevermind I answered my own question. For those of you that need to know...
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PS 2 consoles with version 4 boards may NOT be able to boot PSX backups using the traditional method. To play PSX backups on a v4 PS2, boot normally, but once you reach the startup screen ("Please Insert A Playstation CD"), press the RESET button once. The backup should now work.
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This method worked for me.