  After the second data release of Gaia, the number of new globular cluster
candidates has increased importantly. However, most of them need to be properly
characterised, both spectroscopically and photometrically, by means of radial
velocities, metallicities, and deeper photometric observations. Our goal is to
provide an independent confirmation of the cluster nature of Gran 4, a recently
discovered globular cluster, with follow-up spectroscopic observations. The
derived radial velocity for individual stars, coupled with proper motions,
allows us to isolate cluster members from field stars, while the analysis of
their spectra allows us to derive metallicities. By including in the analysis
the recently confirmed clusters Gran 1, 2, 3, and 5, we aim to completely
characterise the sample presented in Gran et al. 2022. Using Gaia DR3 and VVV
catalogue data and MUSE@VLT observations, we performed a selection of cluster
members based on their proper motions, radial velocities and their position in
colour-magnitude diagrams. Furthermore, full spectral synthesis was performed
on the cluster members, extracting surface parameters and metallicity from MUSE
spectra. Finally, a completeness estimation was performed on the total globular
cluster population of the Milky Way. We confirm the nature of Gran 4, a newly
discovered globular cluster behind the Galactic bulge, with a mean radial
velocity of ${\rm RV} = -265.28 \pm 3.92$ km s$^{-1}$ and a mean metallicity of
${\rm [Fe/H] = -1.72 \pm 0.32}$ dex. Additionally, independent measurements of
the metallicities were derived for Gran 1, 2, 3, and 5. We also revise the
observational lower mass limit for a globular cluster to survive in the
bulge/disk environment. We estimate that $\sim 12-26$ globular clusters have
still to be discovered on the other side of the Galaxy (i.e., behind the
bulge/bar/disk), up to 20 kpc.
