Component of the 26S proteasome, a multiprotein complex involved in the ATP-dependent degradation of ubiquitinated proteins. This complex plays a key role in the maintenance of protein homeostasis by removing misfolded or damaged proteins, which could impair cellular functions, and by removing proteins whose functions are no longer required. Therefore, the proteasome participates in numerous cellular processes, including cell cycle progression, apoptosis, or DNA damage repair. PSMD4 acts as an ubiquitin receptor subunit through ubiquitin-interacting motifs and selects ubiquitin-conjugates for destruction. Displays a preferred selectivity for longer polyubiquitin chains. Component of the 19S proteasome regulatory particle complex. The 26S proteasome consists of a 20S core particle (CP) and two 19S regulatory subunits (RP). The regulatory particle is made of a lid composed of 9 subunits, a base containing 6 ATPases and few additional components including PSMD4 (PubMed:27428775, PubMed:27342858). Interacts with NUB1 (PubMed:11585840). Interacts with SQSTM1 (PubMed:15340068). Interacts with UBQLN4 (PubMed:15280365). Interacts with UBE3A (PubMed:22645313). Interacts with UBQLN1 (via ubiquitin-like domain) (PubMed:15147878). Interacts with DDI2 (PubMed:29290612). Additional isoforms seem to exist. The 2 UIM motifs are involved in the binding to a multi-ubiquitin chain in a cooperative way. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. Belongs to the proteasome subunit S5A family.