A tetra-component blend, consisting of low-density polyethylene (LDPE), polyvinyl chloride (PVC), polypropylene (PP), and polystyrene (PS), was studied as a model system of commingled plastic wastes (LDPE/PVC/PP/PS, mass ratio: 70/10/10/10). Effects of chlorinated polyethylene (CPE), ethylene-propylene-diene monomer (EPDM), styrene-butadiene-styrene (SBS), and their mixture (CPE/EPDWSBS, mass ratio: 2/2/2) on the mechanical properties and morphology of the system were investigated. With addition of several elastomers and their mixture, the tensile strength of the blends decreased slightly, although both the elongation at break and the impact strength increased. Among these elastomers, EPDM exhibited the most significant impact modification effect for the tetra-component blends. SBS and the mixture have a good phase-dispersion effect for the tetra-component blend. By adding a crosslinking agent [dicumyl peroxide (DCP)], the mechanical properties of the tetra-component blends also increased. When either SBS or the mixture was added to the blend together with DCP, the probability that the crosslinking agent (DCP) would be at the interface improved because of the phase-dispersion effect of SBS. Therefore, more co-crosslinked products will form between LDPE and other components. Accordingly, remarkable improvement of the interfacial adhesion and hence the mechanical properties of the tetra-component blends occurred.