III's were trained from a very young age, I think 10. IV's are just super awesome Marines, but went through very rigorous tests/training.
A SIII would win against an SIV due to the significantly longer training time they had in a fist to fist fight but a SIV would win against a SIII in an armed battle due to the IV's having the far better armor. III's are probably smarter but I'm sure both have been taught tactics and strategies.
You are the child of my makers. Inheritor of all they left behind. You are Forerunner! But this ring... is mine.