  Gamma-ray emission provides constraints on the non-thermal radiation
processes at play in astrophysical particle accelerators. This allows both the
nature of accelerated particles and the maximum energy that they can reach to
be determined. Notably, it remains an open question to what extent supernova
remnants (SNRs) contribute to the sea of Galactic cosmic rays. In the Galactic
plane, at around 312{\deg} of Galactic longitude, Fermi-LAT observations show
an extended source (4FGL J1409.1-6121e) around five powerful pulsars. This
source is described by one large disk of 0.7{\deg} radius with a high
significance of 45 sigma in the 4FGL-DR3 catalog. Using 14 years of Fermi-LAT
observations, we revisited this region with a detailed spectro-morphological
analysis in order to disentangle its underlying structure. Three sources have
been distinguished, including the supernova remnant G312.4-0.4 whose gamma-ray
emission correlates well with the shell observed at radio energies. The hard
spectrum detected by the LAT, extending up to 100 GeV without any sign of
cut-off, is well reproduced by a purely hadronic model.
