  This paper presents the first study that uses the GEometry ANd Tracking 4
(GEANT4) toolkit to do quantitative comparisons with other modelling results
related to the production of Terrestrial Gamma-ray Flashes (TGFs) and
high-energy particle emission from thunderstorms. We will study the
Relativistic Runaway Electron Avalanche (RREA) and the relativistic feedback
process, as well as the production of bremsstrahlung photons from Runaway
Electrons (REs). The Monte Carlo (MC) simulations take into account the effects
of electron ionisation, electron by electron (M{\o}ller) and electron by
positron (Bhabha) scattering as well as the bremsstrahlung process and
pair-production, in the $250$ eV$-100$ GeV energy range. Our results indicate
that the multiplication of electrons during the development of RREAs and under
the influence of feedback, are consistent with previous estimates. This is
important to validate GEANT4 as a tool to model RREAs and feedback in
homogeneous electric fields. We also determine the ratio of bremsstrahlung
photons to energetic electrons $N_\gamma/N_e$. We then show that the ratio has
a dependence on the electric field which can be expressed by the avalanche time
$\tau(E)$ and the bremsstrahlung coefficient $\alpha(\varepsilon)$. In
addition, we present comparisons of GEANT4 simulations performed with a
"standard" and a "low-energy" physics list both validated in the $1$ keV$-100$
GeV energy range. This comparison shows that the choice of physics list used in
GEANT4 simulations has a significant effect on the results.
