Given that you generally play as Master Chief, why on hell would you expect a Next-Gen SPARTAN be scripted to totally eclipse you?
The core difference is that Pre-Gens were involuntarily inducted into the program. Then, volunteers. Then, volunteers with previous actual combat records pre-induction (post-War).
That's it. If you actually take out of play how support-characters are depicted for the sake of story-progression, there's NO REASON to think that the IVs are inferior to the IIs.
In fact, them being IVs contributes to them being "meh" because the line between "non" and I was huge, between "I" and "II" significant.
It is an error to compare a IV to a II, while comparing IIs to "non".
Math: (# indicates bad-assery)
0 (non-spartan human)
5 (I)
6 (II)
7 (III)
8 (IV)
Using the math example, a SPARTAN II has a gap of "6" over humans. A SPARTAN IV might only have a gap of "2" over the SPARTAIN IIs, but that's a total gap of "8" over humans. Despite how close each SPARTAN Gen may be to its predecessor, that's still a continually UPWARD growth in bad-assery as a species.
They are inherently more bad-ass, even if not depicted as such.