  In the last decade, a dozen close-in giant planets have been discovered
orbiting stars with spectral types ranging from M0 to M4, a mystery since known
formation pathways do not predict the existence of such systems. Here, we
confirm TOI-4860 b, a Jupiter-sized planet orbiting an M4.5 host, a star at the
transition between fully and partially convective interiors. First identified
with TESS data, we validate the transiting companion's planetary nature through
multicolour photometry from the TRAPPIST-South/North, SPECULOOS, and MuSCAT3
facilities. Our analysis yields a radius of $0.76 \pm 0.02~ \rm R_{Jup}$ for
the planet, a mass of $0.34~\rm M_\odot$ for the star, and an orbital period of
1.52 d. Using the newly commissioned SPIRIT InGaAs camera at the
SPECULOOS-South Observatory, we collect infrared photometry in zYJ that spans
the time of secondary eclipse. These observations do not detect a secondary
eclipse, placing an upper limit on the brightness of the companion. The
planetary nature of the companion is further confirmed through high-resolution
spectroscopy obtained with the IRD spectrograph at Subaru Telescope, from which
we measure a mass of $0.67 \pm 0.14~\rm M_{Jup}$ . Based on its overall
density, TOI-4860 b appears to be rich in heavy elements, like its host star.
