Alright, first thing is first, if you take a real close look (break out your reading glasses) at the address of this wiki, you're going to see that the subdomain is "halofanon". Now, for those of you who don't know, Fanon is "Fan" + "Canon" and works like a large community built fanfic. Halo Fanon is just that, a fan-made wiki.
That being said: Wow. These people can seriously write some stuff. I am impressed.
Anyway, yes, there are SPARTAN IV's aboard the UNSC Prototype Infinity. Yes, they are adult voltuneers primarily made up of former ODST and former UNSC Marines, similar to the SPARTAN I's. That much is fact. The IV's, if I remember correctly were an officially sanctioned UNSC pet project of Vice-Admiral Parangosky, and neither Halsey, nor Mendez knew of them. The SPARTAN IV program was started, like one of you said, to bring the SPARTAN program back to it's ORION roots, that is, as a way to covertly and decisively quash colony rebellion.
I also think we're all going to be pleasantly surprised when we find out who trained them, too.