  We report the discovery of the massive hot Jupiter NGTS-13b by the Next
Generation Transit Survey (NGTS). The V = 12.7 host star is likely in the
subgiant evolutionary phase with log g$_{*}$ = 4.04 $\pm$ 0.05, T$_{eff}$ =
5819 $\pm$ 73 K, M$_{*}$ = 1.30$^{+0.11}_{-0.18}$ M$_{\odot}$, and R$_{*}$ =
1.79 $\pm$ 0.06 R$_{\odot}$. NGTS detected a transiting planet with a period of
P = 4.12 days around the star, which was later validated with the Transiting
Exoplanet Survey Satellite (TESS; TIC 454069765). We confirm the planet using
radial velocities from the CORALIE spectrograph. Using NGTS and TESS full-frame
image photometry combined with CORALIE radial velocities we determine NGTS-13b
to have a radius of R$_{P}$ = 1.142 $\pm$ 0.046 R$_{Jup}$, mass of M$_{P}$ =
4.84 $\pm$ 0.44 M$_{Jup}$ and eccentricity e = 0.086 $\pm$ 0.034. Some previous
studies suggest that $\sim$4 M$_{Jup}$ may be a border between two separate
formation scenarios (e.g., core accretion and disk instability) and that
massive giant planets share similar formation mechanisms as lower-mass brown
dwarfs. NGTS-13b is just above 4 M$_{Jup}$ making it an important addition to
the statistical sample needed to understand the differences between various
classes of substellar companions. The high metallicity, [Fe/H] = 0.25 $\pm$
0.17, of NGTS-13 does not support previous suggestions that massive giants are
found preferentially around lower metallicity host stars, but NGTS-13b does
support findings that more massive and evolved hosts may have a higher
occurrence of close-in massive planets than lower-mass unevolved stars.
