ABSTRACT We report the detection of [C ii] λ158 $μ$m emission from a system of three closely separated sources in the Cosmic Evolution Survey (COSMOS) field at z ∼ 4.56 , as part of the Atacama Large Millimeter/submillimeter Array (ALMA) Large Program to INvestigate C ii at Early times (ALPINE). The two most luminous sources are closely associated, both spatially (1.6 arcsec ∼ 11 kpc) and in velocity (∼100 km s−1), while the third source is slightly more distant (2.8 arcsec ∼ 18 kpc, ∼300 km s−1). The second most luminous source features a slight velocity gradient, while no significant velocity gradient is seen in the other two sources. Using the observed [C ii] luminosities, we derive a total log$_{10}(\rm SFR_{[C\,{\small II}]}\, [M_{⊙ }\, yr^{-1}])=2.8± 0.2$, which may be split into contributions of 59, 31, and 10 per cent from the central, east, and west sources, respectively. Comparison of these [C ii] detections to recent zoom-in cosmological simulations suggests an ongoing major merger. We are thus witnessing a system in a major phase of mass build-up by merging, including an ongoing major merger and an upcoming minor merger, which is expected to end up in a single massive galaxy by z ∼ 2.5.