Core histone-binding subunit that may target chromatin assembly factors, chromatin remodeling factors and histone deacetylases to their histone substrates in a manner that is regulated by nucleosomal DNA. Component of several complexes which regulate chromatin metabolism. These include the chromatin assembly factor 1 (CAF-1) complex, which is required for chromatin assembly following DNA replication and DNA repair; the core histone deacetylase (HDAC) complex, which promotes histone deacetylation and consequent transcriptional repression; the nucleosome remodeling and histone deacetylase complex (the NuRD complex), which promotes transcriptional repression by histone deacetylation and nucleosome remodeling; the PRC2 complex, which promotes repression of homeotic genes during development; and the NURF (nucleosome remodeling factor) complex. Binds directly to helix 1 of the histone fold of histone H4, a region that is not accessible when H4 is in chromatin. Subunit of the chromatin assembly factor 1 (CAF-1) complex, which is composed of RBBP4, CHAF1B and CHAF1A (By similarity). Subunit of the core histone deacetylase (HDAC) complex, which is composed of HDAC1, HDAC2, RBBP4 and RBBP7 (By similarity). The core HDAC complex associates with SIN3A, ARID4B/SAP180, SAP18, SAP30, SAP130, SUDS3/SAP45 and possibly ARID4A/RBP1 and ING1 to form the SIN3 HDAC complex (Probable). The core HDAC complex may also associate with MTA2, MBD3, CHD3 and CHD4 to form the nucleosome remodeling and histone deacetylase complex (the NuRD complex) (By similarity). The NuRD complex may also interact with MBD3L1 and MBD3L2 (By similarity). Interacts with MTA1 (By similarity). Component of the PRC2 complex, which consists of the core subunits EED, EZH1 or EZH2, SUZ12, and RBBP4, and various combinations of accessory subunits including AEBP2, JARID2, PHF19, MTF2 and EPOP (PubMed:19026780). Forms a monomeric PRC2.2 (class 2) complex consisting of at least SUZ12, RBBP4, AEBP2 and JARID2 (By similarity). Forms a dimeric PRC2.1 (class 1, PRC-PCL) complex consisting of at least SUZ12, RBBP4, and PHF19; PHF19 stabilizes the dimeric structure which enhances PRC2 interaction with chromatin (By similarity). Component of the NURF-1 ISWI chromatin remodeling complex (also called the nucleosome-remodeling factor (NURF) complex) at least composed of SMARCA1; BPTF; RBBP4 and RBBP7 (PubMed:10866654). Within the complex interacts with SMARCA1 (By similarity). Interacts with the ISWI chromatin remodeling complex component SMARCA5; the interaction is direct (By similarity). Interacts with SUV39H1 and HDAC7 (PubMed:10984530, PubMed:11788710). Interacts with the viral protein-binding domain of the retinoblastoma protein (RB1) (By similarity). Interacts with SPEN/MINT (By similarity). Interacts with BRCA1 (By similarity). Interacts with CREBBP, and this interaction may be enhanced by the binding of phosphorylated CREB1 to CREBBP (By similarity). Component of the DREAM complex (also named LINC complex) at least composed of E2F4, E2F5, LIN9, LIN37, LIN52, LIN54, MYBL1, MYBL2, RBL1, RBL2, RBBP4, TFDP1 and TFDP2 (By similarity). The complex exists in quiescent cells where it represses cell cycle-dependent genes (By similarity). It dissociates in S phase when LIN9, LIN37, LIN52 and LIN54 form a subcomplex that binds to MYBL2 (By similarity). Interacts with PHF6 (By similarity). Found in a complex composed of at least SINHCAF, SIN3A, HDAC1, SAP30, RBBP4, OGT and TET1 (PubMed:28554894). Interacts with ERCC6 (By similarity). Interacts with ZNF827; the interaction is direct and recruits RBBP4 to telomeres (By similarity). Interacts with ARMC12 (via ARM domains) (By similarity). Interacts with PWWP2B (PubMed:34180153). Localizes to chromatin as part of the PRC2 complex. Higher levels in brain, thymus, lung, spleen, kidney, testis, and ovary/uterus; lower levels in heart, liver, and muscle. Belongs to the WD repeat RBAP46/RBAP48/MSI1 family.