Is the Super Saiyan 4 transformation from Dragon Ball GT confirmed non-canon?
He didn't confirm it, but he didn't create the design either. It was created by a guy named Nakatsuru. Up to today, in Dragon Ball, canon is considered what is based on Akira Toriyama's original ideas. The 80's and 90's movies weren't canon because Akira Toriyama wasn't involved in them. The 80's manga was canon because it was drawn by him. The fillers in the 90's series weren't canon because they weren't based on Akira Toriyama's ideas. So yes, you can say it isn't canon, that doesn't necessarily mean it is completely ruled out, though most likely it is. The final kamehameha was an idea from the video games which wasn't canon, yet it still appeared later in the Dragon Ball super series.