The very tricky aspects of inertia loaded cycling aside there is ZERO physiological benefit to training with heavy bike, wheels, backpack, whatever.
You may think you're faster, and because of the weight reduction maybe you are but if you want to train to ride faster... then ride faster.
Well if he rides those 4kg wheels uphill on his local hill, won't it be better leg strength/fatigue training than using a lighter bike?
(Assuming he goes up the hill, then rolls down, repeats 10-18 times a set?)
Assumes that general fatigue will have an effect on a specific performance and that more leg strength is going to have an effect on endurance performance. One could not make the argument that a higher squat or deadlift will equal better performance in the same way as power to weight on a hilly course or power to frontal area on a flat course.