  We recently introduced new concepts, implemented in EPOS4, which allow to
consistently accommodate factorization and saturation in high energy
proton-proton and nucleus-nucleus collisions, in a rigorous parallel scattering
framework. EPOS4 has a modular structure and in this paper, we present in
detail how the "single scattering module" (the main EPOS4 building block) is
related to perturbative QCD, and how these calculations are performed, with
particular care being devoted to heavy flavor contributions. We discuss
similarities and differences compared to the usual pQCD approach based on
factorization.
