  We present an ALMA high-resolution observation of the 840 um continuum and
[CII] line emission in the WISE-SDSS selected hyper-luminous (WISSH) QSO
J1015+0020 at z~4.4. Our analysis reveals an exceptional overdensity of
[CII]-emitting companions with a very small (<150 km/s) velocity shift with
respect to the QSO redshift. We report the discovery of the closest companion
observed so far in submillimetre observations of high-z QSOs. It is only 2.2
kpc distant and merging with J1015+0020, while two other [CII] emitters are
found at 8 and 17 kpc. Two strong continuum emitters are also detected at <3.5
arcsec. They are likely associated to the same overdense structure of
J1015+0020, as they exceed by a factor of 100 the number of expected sources,
considering the Log(N)-Log(S) at 850 um. The host galaxy of J1015+0020 shows a
SFR of about 100 Msun/yr while the total SFR of the QSO and its companion
galaxies is a factor of 10 higher, indicating that substantial stellar mass
assembly at early epochs may have taken place in the QSO satellites. For
J1015+0020 we compute a SMBH mass MBH~6E9 Msun and a dynamical mass Mdyn~4E10
Msun . This translates into an extreme ratio Mdyn/MBH~7. The total stellar mass
of the QSO host galaxy plus the [CII] emitters already exceeds 1E11 Msun at
z~4.4. These sources will likely merge and develop into a giant galaxy of
1.3E12 Msun. Under the assumption of constant mass or Eddington accretion rate
equal to the observed values, we find that the growth timescale of the host
galaxy is comparable or even shorter than that inferred for the SMBH.
