RNA-binding protein which binds to pre-mRNA, spliced mRNA and spliceosomal snRNAs (PubMed:14706697, PubMed:23149939). Preferentially binds to the spliceosomal snRNP U2 (PubMed:23149939). Core component of the splicing-dependent multiprotein exon junction complex (EJC) deposited at splice junctions on mRNAs (By similarity). Positively regulates the nuclear export of spliced mRNAs including the sex determination gene tra-2 (PubMed:23149939). Promotes oogenesis by repressing germline masculinization and spermatogenesis (PubMed:14706697). Required for embryonic development, possibly through positively regulating the expression of pie-1 (PubMed:14706697, PubMed:30279189). Not required for the spatial patterning of pie-1 in embryos (PubMed:30279189). Heterodimer with mag-1/MAGOH (PubMed:14706697). Part of the mRNA splicing-dependent exon junction complex (EJC) complex; the core complex contains CASC3, EIF4A3, mag-1/MAGOH and rnp-4/RBM8A (Probable). Interacts with rsr-1/SRm160 (PubMed:12944400). Expressed in somatic cells and germline cells. First expressed in 20-30 cell stage embryos and expression continues through to adulthood. RNAi-mediated knockdown results in embryonic lethality due to developmental arrest during morphogenesis (PubMed:30279189). RNAi-mediated knockdown at the L1 larval stage results in increased sperm production and no oocyte production (PubMed:14706697). RNAi-mediated knockdown at the L1 larval stage results in the accumulation of unspliced mRNAs including ama-1 and ife-4 in the cytoplasm, and specifically in the production of an irregular unspliced form of the sex determining protein tra-2, which accumulates in the cytoplasm and inhibits the regular tra-2 isoforms a and c reducing their expression and disrupting their nuclear localization (PubMed:23149939). RNAi-mediated knockdown in L4 hermaphrodite larvae results in 90% embryonic lethality with embryos exhibiting elongation defects (PubMed:14706697). RNAi-mediated knockdown reduces the expression of pie-1 in P2 blastomeres (PubMed:30279189). RNAi-mediated knockdown together with nxf-2 RNAi results in severe growth retardation, lethality and reduced accumulation of unspliced tra-2 in the cytoplasm (PubMed:23149939). RNAi-mediated knockdown together with ire-1 RNAi restores fertility, reduces the accumulation of unspliced tra-2 in the cytoplasm and suppresses germ line masculinization of the single rnp-4 RNAi mutant (PubMed:23149939). Belongs to the RBM8A family.