"Halo 4: I Get a Woman!" - Sgt. Mjr. A.J. Johnson, RIP

[Zeph]

SPARTAN-IV's? Ahahahaha! Oh, that's too funny. Blah blah blah, we are 343i, blah blah blah, Spartan IV's are superduperawesomefun, blah blah blah...

343i must not like what Nylund did to the Spartan-III's. If you're going to go non-canon, why the fuck not just use regular SPARTAN-II's?

No, their idea behind Spartan IVs is along the lines of the following:

Spartans held a physical limit on what their bodies could endure.
Spartan IIs overcame this limit through genetic enhancement. Experimental and high-budget prototype stage.
Spartan IIIs took lessons learned through Spartan IIs and applied them on a much larger scale. Lower budget test stage.
Spartan IVs are the perfection of the technique. Cheap as possible and pools candidates from entire human population.

edit: It's not about one being better or worse than another. This simply has to do with what point it is on the Halo timeline.
/edit

I don't know about the third thing :P

The infinitely variable transmission part was the whole reason the warthog had a suspension the way it did and "one" gear.