Not considered sequels

[landman]

The main villain of the original series - The Profound Darkness - has been vanquished for good in IV. The story has been wrapped up back in 1993.

And yet, Phantasy Star 3 takes place a millennia after Phantasy Star 4, PD away or not, little falzes are still around destroying spaceships, being sealed by swords, and unsealed to be fought again. The original chronology (that is, the Japanese one, English games were mistranslated) of the classic series is not closed at all, if a sealed Falz in the Alisa 3 survived when PD was destroyed, other falzes could too, just like... a certain falz sealed into a spaceship that was buried on Ragol, with monolith's texts describing 1000 years resurrection cycles and with an image at the ending that hint's an esper like figure sealing falz in a monolith using the psychowand.