My personal configuration in-game was chosen based on what looked like it would provide the greatest range of motion. The Wetworks chest seems to be the least restrictive, particularly as the sides appear to be attached via cables rather than being solid straps. In-game I tend to shift between ODST, Counter, and Contoured legs as the mood hits, Mk V forearms, although contoured and inner-plated also have fairly low profiles, thus less weight and protrusions. Shoulders, again I have three I swap around, the Soldier, War Master, and Mark V models.
But as you've observed, the overall design of the Spartan IV armor is more mobility-focused (main improvement being the omission of the "space diaper" of old). So really it comes down to how you build the armor. Some have gone the "apron/poncho" approach, putting all the chest, abdominal, cod, and in some cases even shoulder armor all together on the same piece following the outline of the two-toned undersuit, while others have put in the extra work to actually make the two-toned undersuit and attach the hard armor as individual segments, thus allowing more freedom of movement. The way the in-game model is set up, the main torso armor covers the rib cage (which doesn't flex much even with rigorous movements), secondary plating directly over the abdomen (with sufficient spacing that it will not rub against the chest plating or come in contact with the cod piece, short of curling into a ball). Arm and leg armor for Spartan IVs has been "trimmed" back compared to the Halo 1-3 models, which offered very little separation around the joints. And even the animation of movement takes hard points into consideration more, while Halo 1-3 required a lot of clipping to allow for full movement. It was once anecdotaly observed that if someone wore a 100% proportionately accurate Mark V/VI suit, they would be almost entirely unable to move. Reach's take on the Mark V opened up a lot more mobility, and Halo 4 took that a step further with all of their armor.
