I’m guessing that unlike “Infinty”, Spartan Ops will take place in actual combat situations. Since Halo 4 is supposed to explain the UNSC during the post-war perioid, and since John is away on an unknown Forerunner world, I’m guessing that Spartan Ops is where we learn the most of what is going on with humanity.
By the way, how does John have access to a newer variant of UNSC weapons? I’m going to make a far-fetch claim and guess that the UNSC Infinity might be sending him state-of-the-art supplies via Slipspace, but they are unable to reach John physically because, hell I dunno, Rainbow Sentinels that shoot chainsaws are patrolling the planet?
It was said somewhere that the UNSC Infinity crosses paths with John during the campaign. So that would explain where he gets new weapons, vehicles and UNSC troops. Hopefully this means John will meet up with whats left of his old Spartan team as they are getting integrated with the Spartan 4’s. 
I’m going to guess that if the Covenant can find him, the UNSC can too. I think we’ll see them in the first mission.
It’s a pretty sure thing that the Covenant had forces in places we’ve never been. Not much is left of the Covenant at this point, but those forces could certainly remain. We know from Glasslands that some grunts allied with the Elites, and that the Brutes are a bit of a wildcard now. The images we saw of the Sangheili in GI are also telling. The armor is well-worn or missing entire pieces, so John’s not likely dealing with anything on the scale of the military machine we’ve known the Covenant to be so far. Could be scavengers or (my bet) the Servants of the Abiding Truth. In any case, it doesn’t mean that “the Covenant” have found him (whatever the Covenant means any more - very little, to me). And it’s not a sure bet that the UNSC has located him, either.
That’s not to say I don’t think they’l find him, even very early in the game.
> It was said somewhere that the UNSC Infinity crosses paths with John during the campaign. So that would explain where he gets new weapons, vehicles and UNSC troops. Hopefully this means John will meet up with whats left of his old Spartan team as they are getting integrated with the Spartan 4’s.
I’ve seen a couple of screenshots of the first mission of Halo 4, and in it, John seems to be aboard the remains of the Dawn, fighting Elites and Grunts with the new BR85 and the new variant of the MA5 already in hand.