You're welcome but I wouldn't say Ys IV is superior to Ys Book I&II, and I mostly love them all given the legendary soundtracks.
Well, opinions differs, I understand.
Well, for me, in Ys I & II except few memorable music tracks (for example Fields, Boss music and Belfry section) I see nothing outstanding here.
In DoY I've liked much more the world, dungeons, characters, story (more sh*t happens around too, got suprised with some events too).
Most of soundtrack is damn good.
The graphics are superior.
Controls were improved, diagonal movement finally here.
Much more content.
Much more varied bestiary (and many magic-ressistance enemies finally).
Most step up are bossfights in my opinion, they've ashamed those seen in Ys I II. Finally varied movesets, more intense action, and that 3 phase final boss with that damn good soundtrack.
For me it is just bigger and better than ys I & II