  The EPOS4 project is an attempt to construct a realistic model for describing
relativistic collisions of different systems, from proton-proton ($pp$) to
nucleus-nucleus ($AA$), at energies from several TeV per nucleon down to
several GeV. We argue that a parallel scattering formalism (as in EPOS4) is
relevant for primary scatterings in AA collisions above 4 GeV, whereas
sequential scattering (cascade) is appropriate below. We present briefly the
basic elements of EPOS4, and then investigate heavy ion collisions from 62.4
GeV down to 4 GeV, to understand how physics changes with energy, studying in
particular the disappearance of the fluid component at low energies.
