  The electromagnetic responses obtained from Green's function Monte Carlo
(GFMC) calculations are based on realistic treatments of nuclear interactions
and currents. The main limitations of this method comes from its
nonrelativistic nature and its computational cost, the latter hampering the
direct evaluation of the inclusive cross sections as measured by experiments.
We extend the applicability of GFMC in the quasielastic region to intermediate
momentum transfers by performing the calculations in a reference frame that
minimizes nucleon momenta. Additional relativistic effects in the kinematics
are accounted for employing the two-fragment model. In addition, we developed a
novel algorithm, based on the concept of first-kind scaling, to compute the
inclusive electromagnetic cross section of $^4$He through an accurate and
reliable interpolation of the response functions. A very good agreement is
obtained between theoretical and experimental cross sections for a variety of
kinematical setups. This offers a promising prospect for the data analysis of
neutrino-oscillation experiments that requires an accurate description of
nuclear dynamics in which relativistic effects are fully accounted for.
