  Charge carrier mobility is at the core of semiconductor materials and devices
optimization, and Hall measurement is one of the most important techniques for
its characterization. The Hall factor, defined as the ratio between Hall and
drift mobilities, is of particular importance. Here we study the effect of
anisotropy by computing the drift and Hall mobility tensors of a
technologically important wide-band-gap semiconductor, 4H-silicon carbide
(4H-SiC) from first principles. With $GW$ electronic structure and \textit{ab
initio} electron-phonon interactions, we solve the Boltzmann transport equation
without fitting parameters. The calculated electron and hole mobilities agree
with experimental data. The electron Hall factor strongly depends on the
direction of external magnetic field $\mathbf{B}$, and the hole Hall factor
exhibits different temperature dependency for $\mathbf{B}\parallel c$ and
$\mathbf{B}\perp c$. We explain this by the different equienergy surface shape
arising from the anisotropic and non-parabolic band structure, together with
the energy-dependent electron-phonon scattering.
