Well at first I'm hoping they'll struggle, this is basically Contact Harvest all over again, Humans encounter a new Alien civilisation, so they're in a rough situation already, no information of what the enemy are, their military structure, their weapon capabilities, weaknesses etcetera.
But we know that the Spartan IV's are compiled of Marines, and I believe ODST's/Spartans from previous programmes as well? (I wonder if the Spartan's that are re-rolled into the Spartan IV programme will be easily recognisable from their marine counterparts in terms of ability and strength, which would be a nice touch).
And with Human's technology advancements, especially the recovery of Forerunner technology from the Onyx shield world, AND the Engineer's from the Onyx shield world, the Spartan IV's will probably not struggle for long, and show the enemy they mean business.
It'll be like the Forerunner-Human wars. It's almost identical for Halo 4's situation, instead of Flood, we have Covenant.
And I...believe I've rambled of and avoided the topic discussion, I do apologise