Luminous infrared galaxies at high redshifts ($z$>4) include extremestarbursts that build their stellar mass over short periods of time (>100 Myr).These galaxies are considered to be the progenitors of massive quiescentgalaxies at intermediate redshifts ($z\sim$2) but their stellar structure andbuildup is unknown. Here, we present the first spatially resolved near-infraredimaging of GN20, one of the most luminous dusty star-forming galaxies known todate, observed at an epoch when the Universe was only 1.5 Gyr old. The5.6$\mu$m image taken with the JWST Mid-Infrared Instrument (MIRI/JWST) showsthat GN20 is a very luminous galaxy (M$_\mathrm{1.1\mu m,AB}$=$-$25.01), with astellar structure composed of a conspicuous central source and an extendedenvelope. The central source is an unresolved nucleus that carries 9% of thetotal flux. The nucleus is co-aligned with the peak of the cold dust emission,and offset by 3.9 kpc from the ultraviolet stellar emission. The diffusestellar envelope is similar in size to the clumpy CO molecular gasdistribution. The centroid of the stellar envelope is offset by 1 kpc from theunresolved nucleus, suggesting GN20 is involved in an interaction or mergerevent supported by its location as the brightest galaxy in a proto-cluster. Thestellar size of GN20 is larger by a factor of about 3-5 than known spheroids,disks, and irregulars at $z\sim$4, while its size and low S\'ersic index aresimilar to those measured in dusty, infrared luminous galaxies at $z\sim$2 ofthe same mass. GN20 has all the ingredients necessary for evolving into amassive spheroidal quiescent galaxy at intermediate $z$: it is a large,luminous galaxy at $z$=4.05 involved in a short and massive starburst centredin the stellar nucleus and extended over the entire galaxy, out to radii of 4kpc, and likely induced by the interaction or merger with a member of theproto-cluster.