We present an ALMA high-resolution (0.18″ × 0.21″) observation of the 840 μm continuum and [CII] λ157.74 μm line emission in the WISE-SDSS selected hyper-luminous (WISSH) quasi-stellar object (QSO) J1015+0020, at z ∼ 4.4. Our analysis reveals an exceptional overdensity of [CII]-emitting companions with a very small (<150 km s⁻¹) 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 from the QSO. They are likely associated with 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 μm. The host galaxy of J1015+0020 shows a star formation rate (SFR) of about 100 M_⊙ yr⁻¹, 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 computed a black hole mass M_BH ∼ 6 × 10⁹ M_⊙. As we resolve the [CII] emission of the QSO, we can compute a dynamical mass of M_dyn ∼ 4 × 10¹⁰ M_⊙. This translates into an extreme ratio M_dyn/M_BH ∼ 7, i.e. two orders of magnitude smaller than what is typically observed in local galaxies. The total stellar mass of the QSO host galaxy plus the [CII] emitters in the ALMA field of view already exceeds 10¹¹ M_⊙ at z ∼ 4.4. These sources will likely merge and develop into a giant galaxy of ∼1.3 × 10¹² M_⊙. Under the assumption of constant Ṁ_acc or λ_Edd equal to the observed values, we find that the growth timescale of the host galaxy of J1015+0020 is comparable or even shorter than that inferred for the SMBH.