West, G. B. / Brown, J. H. / Enquist, B. J. A General Model for the Origin of Allometric Scaling Laws in Biology states

our result generalizes to a 5 D/(D 1 1). Organisms are three-dimensional, which explains the 3 in the numerator of the 3/4 power law

They do not make a suggestion for the 4 in the denominator.

I agree about the 3. So what explains the 4 in the denominator?

For a suggestion:

Preprint Metabolic scaling's homeostasis