It has twice as many playable characters for one
And you have to remember, HS is a new concept, a new engine (unless they used their last game : Kung Fu Chaos) which I doubt, so a lot of pre-development time.
DMC4, 3 years of development is mostly for pure-content I'd imagine. They know the concept (DMC1 / DMC3 kicked ass) they know the design system, they know the art style, they alreay have a pre-built engine, they know the mo-cap actors/VA and Just Cause Productions, they have gone on record saying DMC4 has more enemies/bosses than previous DMC games combined.
The odds of DMC4 having twice as many animations as DMC3 is VERY HIGH. Considering DMC3 in itself IS INSIDE OF DMC4 since Dante in a DMC4 carried over all of his attacks, styles, and weapons from DMC3 and then some. And Nero is all brand new. And they even hinted at 'extra playable' characters after game completion.
As for co-op -- DMC3 had the Doppleganger style co-op, and for one mission two people can co-op play as Dante and Vergil (although taht was offline)