  We employ a fireball model of the GRB explosion to constrain intrinsic and
environmental parameters of four events with good broadband afterglow data;
GRB970508, GRB980329, GRB980703, and GRB000926. Using standard assumptions of
constant circumburst density and no evolution of the fraction of the explosion
energy in the post-shock magnetic field, we investigate the uniformity of the
derived explosion and shock physics parameters among the events. We find
densities that range from those of the ISM to diffuse clouds, energies
comparable to the total GRB gamma-ray energy, collimations from near-isotropy
to 0.04 rad, substantial electron energy fractions of 10-30% with energy
distribution indices of 2.1-2.9 and magnetic energy fractions from 0.2-25%. We
also investigate the level to which the data constrain standard model
assumptions, such as the magnetic field evolution, and the allowed density
profiles of the medium. Fits generally improve slightly with an increasing
magnetic energy fraction epB. Good fits can be produced with epB accumulating
or decaying with the shock strength over the afterglow as epB~gamma^x;
-2<=x<=+1. The data are not very sensitive to increasing density profiles,
allowing good fits even with density ~r^10. Some parameter values change by up
to an order of magnitude under altered assumptions; parameters of even good
fits cannot be taken at face value. The data are sensitive to decreasing
densities; r^-2 profiles may produce reasonable fits; steeper profiles, even
r^-2.5, will not fit the data.
