The Spartan IV's are morons because of crappy writing there is no canon related reason. Sorry.
Yeah but let's take a step backward and talk about them as a design choice from 343i themselves. The purpose of Spartan IV's was to give the player a non chief multiplayer character that fit into the universe, but also handled like Spartan II's. I see the big mistake here, and that was one of the first things 343i set out to do, the unision of multiplayer and single player canon, abandoning treating the multiplayer as am enigmatic, separate universe.
And what an absolutely colossal fucking mistake it was, it is one that will now haunt the series for a long time to come.
This all assumes that there is no further development in the works for the Spartan IVs, and I think the game and Spartan Ops have dropped hints that will be explored.
Halsey can't stand the Spartan IV program for a few different reasons, and she doesn't consider them to be true Spartans. Remember the line "First we taught them to be quiet, then we taught them to be Spartans"? That's some sound advice for Majestic, wouldn't you say?
Why are the Spartan IIs so different, and why is John special among that group? The guy who doesn't even allow his humanity to come to the surface? That thinks that he's a machine? Did John's success come about because he lacked basic humanity and was at his core, broken? These are the very first questions posed in the game, in the opening cinematic, catalyzed in this first chapter of a new trilogy that we know focuses on the Chief's humanity, by way of Cortana's arc of rampancy, and continued by stressing the difference between the Chief and new breed of Spartans in the end cinematic and Spartan Ops.
Seems clear to me that this is a direction that 343 may follow up on since they've laid a foundation for it. That the Spartan IVs are new blood that haven't yet learned what it is to be a Spartan like the Chief.