Groups are the most common association of galaxies in the Universe and they are found in different configuration states, such as loose, compact, and fossil groups. We studied the galaxy group MKW 4s, dominated by the giant early-type galaxy NGC 4104 at z=0.0282, with the aim of understanding the evolutionary stage of this group and to place it within the framework of the standard {Lambda}CDM cosmological scenario. We obtained deep optical data with CFHT/Megacam (g and r bands) and we applied both the galfit 2D image fitting program and the IRAF/ellipse 1D radial method to model the brightest group galaxy (BGG) and its extended stellar envelope. We also analysed the publicly available XMM-Newton and Chandra X-ray data. From N-body simulations of dry-mergers with different mass ratios of the infalling galaxy, we were able to constrain the dynamical stage of this system. Our results show a stellar shell system feature in NGC 4104 and an extended envelope that was reproduced by our numerical simulations of a collision with a satellite galaxy taking place about 4-6Gyr ago. The initial pair of galaxies had a mass ratio of at least 1:3. Taking into account the stellar envelope contribution to the total r band magnitude and the X-ray luminosity, MKW 4s falls into the category of a fossil group. Our results show that we are witnessing a rare case of a shell elliptical galaxy in a forming fossil group.