  The study of high contrast imaged brown dwarfs and exoplanets depends
strongly on evolutionary models. To estimate the mass of a directly imaged
substellar object, its extracted photometry or spectrum is used and adjusted
with model spectra together with the estimated age of the system. These models
still need to be properly tested and constrained. HD 4747B is a brown dwarf
close to the H burning mass limit, orbiting a nearby, solar-type star and has
been observed with the radial velocity method over almost two decades now. Its
companion was also recently detected by direct imaging, allowing a complete
study of this particular object. We aim to fully characterize HD 4747B by
combining a well constrained dynamical mass and a study of its observed
spectral features in order to test evolutionary models for substellar objects
and characterize its atmosphere. We combine the radial velocity measurements of
HIRES and CORALIE taken over two decades and high contrast imaging of several
epochs from NACO, NIRC2 and SPHERE to obtain a dynamical mass. From the SPHERE
data we obtain a low resolution spectrum of the companion from Y to H band, as
well as two narrow band-width photometric measurements in the K band. A study
of the primary star allows in addition to constrain the age of the system as
well as its distance. Thanks to the new SPHERE epoch and NACO archival data
combined with previous imaging data and high precision radial velocity
measurements, we have been able to derive a well constrained orbit. We derive a
dynamical mass of mB=70.0$\pm$1.6 MJup which is higher than a previous study,
but in better agreement with the models. By comparing the object with known
brown dwarfs spectra, we derive a spectral type of L9 and an effective
temperature of 1350$\pm$50 K. With a retrieval analysis we constrain the oxygen
and carbon abundances and compare them with the ones from the HR 8799 planets.
