[@Burning Kitty]

The IV are the worst. Without a doubt I would say III are still better than IV even with their advanced technology if put face to face. I goes like this:

Marines=>ODST=>IV=>I=III=>II  if they all had the same gear.

The biggest reasons is the IVs are just normal people given some cool tech, very minor augments, and advanced training. Using technology to get just close as the other two programs. As stated on the wiki "Still cannot produce similar results without risk of a high failure rate, due to the lack of genetic requirements. This drawback is reconciled by modifying the individual to the [u]closest margin possible of the [b]two previous Spartan [/b]programs [/u]". IIIs are not legendary unstoppable beast's as the IIs with dangerous augmentation and trained since children. The IIIs were still trained when still children and receive light augmentation, but didn't get the fancy tech(Besides Noble which were basically Spartan IIs) which is what truly set them apart. They were bullets, meant to engage the target and not return. 

TLDR: Spartan IIIs are close to IIs if equipped correctly. Spartans IV are only better than IIIs because of their new tech since they live in a separate time. A III with Gen 2 armor is almost as good as a Spartan II with Gen 1 . Spartan II with Gen 1 still trump all regardless.

Lots of [i]I [/i] in this post.

EDIT: What I put here is all fact and there is lot of Variables that make them better or worse than each other. Like the Spartan II are all old folks now. Way out of their Prime. The IIIs for the most part never got Mjolnir and are old at this point as well. The IV's currently are the 'best' because they're just more new. But if we compare them all with the same thing. The scaling list is accurate.