Oh, I didn't mean they were working on the game for five years, quite the opposite.
PW2 and PW3 were made one right after the other, they just had to build on top of the building blocks they already have. It was easier to make a sequel and make it feel bigger.
For this one they took a long break, they had to go back and see what they could still use and rebuild half of it.
I'm guessing they updated the character models they already had and picked up new textures and effects more to the speed of a PS4. And I'm pretty sure they're using the same animations they already had, Luffy's moveset, although reinvigorated, looks about the same. Jimbe also showed a couple of repeats, like his special. And I'm pretty sure that Reiju's musou animation is the same Boa Hancock uses in her Love Arrow musou in PW.
But that stills leaves out one third or maybe half of the current moveset, which was heavily altered, to re-do.