Of course not. Ys IV Mask of the Sun is and has always been the original, canon Ys IV. Taito came up with whatever in their own Ys IV, and that's just their crap. The scenario in Ys IV Mask of the Sun SFC were made by Falcom themselves.
Sorry for bringing up an old thread, but I just came across this. Long time fan of Ys, since the Book I and II for the TGFX.
But Dawn of Ys was actually always considered Canon, because it came out first. And Both Dawn and Mask were created by Falcom. They just created a loose framework for the game, and then pitched it to other developers who didn't work together (Tonkin House had actually done previous ports to consoles for Falcom, of the previous Ys games).
It wasn't until later when there was a collection released by Falcom, that they included Mask instead of Dawn, which made Mask canon.
I'm just a big nerd fan for the Ys franchise, especially the music.