The Spartan IV's appear to have a height taller than S-III's, but still shorter than S-II's. I'm judging based off of the height comparisons between Halsey and Nobel Team, and her and the S-IV's during Spartan Ops. Halsey was shoulder height to the S-III's in Noble Team and is slightly below shoulder height to the S-IV's.
Excluding the possibility of simple height inaccuracies across the canon, there are 2 possibilities for the increased height: 1. The new augmentations give a slight increase to both muscle mass and bone size. I'm not sure if the bone part is realistically feasible though. I'm not a biologist or doctor. 2. The new armor could add an additional inch or so to their height compared to the S-III's.
"Jack Tretton, chief executive of Sony Computer Entertainment America, says he is pleased to see so many unsold PS3s sitting on shelves." -IGN