4in vs 4.5in bolt circle rear axle

I would say the stud size is the main issue to be concerned about. Not just the shear strength but the swage surface. It is true that (given the same wheel/tire diameter) the smaller bolt circle experiences higher shear stress on each stud at launch, but the difference is small. Combined with the smaller studs, though, it would definitely be weaker. As far as the rear end, I don't hear a lot about 8-3/4s breaking in street use. It is overall one of the stronger rears out there.