  Groups are the most common association of galaxies in the Universe, found in
different configuration states such as loose, compact and fossil groups. We
have studied the galaxy group MKW 4s, dominated by the giant early-type galaxy
NGC 4104 at z=0.0282. Our aim was to understand the evolutionary stage of this
group and to place it within the framework of the standard LambdaCDM
cosmological scenario. We have obtained deep optical data with CFHT/Megacam (g
and r bands) and we have 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 have also analysed 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 could 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 about 4--6 Gyr 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.
