To answer this question completely you need to read “Glasslands” by Karen Traviss.

However, I’ll explain it in part here.

First, it’s a natural evolution for the Spartan program to proceed to the development of new Spartans. Master Chief is a Spartan II, while Noble Team from Reach, which the exception of Jorge (who himself is also a Spartan II), are all Spartan IIIs.

After the war with the Covenant there are only a handful of Spartan IIs left. There are some Spartan IIIs as well.

Oni takes control of the Spartan III program since Colonel Ackerson (the creator of the Spartan III program) is killed in action during the war. The remaining Spartan IIIs are essentially then to converted to Spartan IVs. For all intents and purposes of this writing it looks like for them it’s an armor swap.

The remainder of the Spartan IVs are to conscripts from the UNSC military, much of which will comes from the ODST ranks, though some may be regular Marines or Army. This leads to a possibility that Gunnery Sergeant Buck may finally become a Spartan. I think that would be a GREAT way to reintroduce Nathan Fillion back into the franchise.

Now, with the addition of Spartan IVs in to the Haloverse, this begs the question, how strong will they be? Obviously Spartan IIIs who convert to Spartan IVs will be stronger than their ODST/Marine/Army converted counterparts. So how will that play out in Multiplayer if at all?

If it does have an affect, I’d say that through playing multiplayer you gain skill and experience. This in turn allows you to rank up from being a marine or army conscript to an ODST conscript to eventually a Spartan III conscript. So by prolific and skillful multiplayer gaming you become stronger and faster. Kind of interesting to think about. This is theoretical on my part though.