Let’s look at the facts.

Spartan IIs:
Superior Training – Taken from early childhood and training for 10 years before being put on the battlefield. Any remaining left as of Halo 4 have more than 40 years of combat experience and have not shown signs of slowing down.
Surgically altered bone mass – Not only are their bones longer making them taller, they are also denser making them much harder to break.
Mental alteration – Let’s not forget they can plug things into their brain (ala Cortana)
Mjolnir suit is stronger (we can run and gun H3-games after you need health packs to pick up)

Spartan IV’s
Spartan IV’s come from the remaining Spartan IIIs, some ODSTs, and some marines. TO this end there is a variance in initial training.
Even when all SIV candidates take on Spartan Training, there will still be that variance for them to fall back on.
SIIIs who have become SIVs were mentally altered and some not for the good.
None of the SIVs have had surgically altered bones, making them normal humans.
Mjolnir suit is not as strong as SIIs. It can’t be. Due to the Surgical enhancements to SIIs the suits can whip their bodies around with a force that would injure or kill those not enhanced. SIV armor has to rely on shields more than SIIs. Healthpacks needed for SIVs have not yet been confirmed that I know of. If so then it points to them being weaker yet again.

Clearly SIVs are not the same as SIIs and are weaker.

The program run by Dr. Catherine Halsey was extremely expensive and secretive. The Spartan II program experimented on the recruits when they were kids, of which roughly half survived the full process to mature SIIs.

SIVs are already grown men and women. They are (as we are led to believe) beyond the age of being surgically enhanced to match the size and strength of SIIs.

Thankfully, we should be at the end of Spartans being weaker with each iteration. Now that they are being formed out of ODST and Marine ranks, really we have nowhere else to go but UP from this point.

With the next iteration of Spartans we may see a similar project as Halsey’s with kids being used again, or at least starting with early teens.

IMO, Master Chief will always be the strongest. If not in pure physical strength, in mental strength, experience, and of course his all important LUCK.

-HFFL