Plays a central role during spermatogenesis by repressing transposable elements and preventing their mobilization, which is essential for the germline integrity (PubMed:17395546, PubMed:18381894, PubMed:18922463, PubMed:26669262, PubMed:22020280). Acts via the piRNA metabolic process, which mediates the repression of transposable elements during meiosis by forming complexes composed of piRNAs and Piwi proteins and governs the methylation and subsequent repression of transposons (PubMed:17395546, PubMed:18381894, PubMed:18922463, PubMed:26669262, PubMed:22020280). Directly binds piRNAs, a class of 24 to 30 nucleotide RNAs that are generated by a Dicer-independent mechanism and are primarily derived from transposons and other repeated sequence elements. Associates with secondary piRNAs antisense and PIWIL2/MILI is required for such association (PubMed:17395546, PubMed:18381894, PubMed:18922463, PubMed:26669262, PubMed:22020280). The piRNA process acts upstream of known mediators of DNA methylation (PubMed:17395546, PubMed:18381894, PubMed:18922463, PubMed:26669262, PubMed:22020280). Does not show endonuclease activity (PubMed:22020280). Plays a key role in the piRNA amplification loop, also named ping-pong amplification cycle, by acting as a 'slicer-incompetent' component that loads cleaved piRNAs from the 'slicer-competent' component PIWIL2 and target them on genomic transposon loci in the nucleus (PubMed:22020280). In addition to its role in germline, PIWIL4 also plays a role in the regulation of somatic cells activities. Plays a role in pancreatic beta cell function and insulin secretion (By similarity). Involved in maintaining cell morphology and functional integrity of retinal epithelial through Akt/GSK3alpha/beta signaling pathway (By similarity). Interacts with PRMT5 and WDR77 (PubMed:19584108). Interacts (when methylated on arginine residues) with TDRD1, TDRKH/TDRD2 and TDRD9 (PubMed:19584108, PubMed:23714778). Interacts with MOV10L1 (PubMed:20534472, PubMed:20547853). Interacts with TEX15 (PubMed:32719317). Interacts with SPOCD1 (PubMed:32674113). Probable component of the meiotic nuage, also named P granule, a germ-cell-specific organelle required to repress transposon activity during meiosis. PIWIL2/MILI is required for nuclear localization. Detected in male germ cells beginning around 14.5-15.5 dpc but is absent from female germ cells (PubMed:18922463, PubMed:25038252). From 15.5 dpc and until birth, it is present in male germ cells. Not detected in somatic cells of the embryonic gonad. Expression declines soon after birth, reaching undetectable levels in 4 day-old mice (at protein level). Highly expressed in male embryonic germ cells at embryonic day 16.5 and expression decreases by postnatal day 2.5. Arginine methylation by PRMT5 is required for the interaction with Tudor domain-containing protein (TDRD1, TDRKH/TDRD2 and TDRD9) and subsequent localization to the meiotic nuage, also named P granule. Males show spermatogenesis defects due to demethylation and subsequent derepression of transposable elements. Meiotic-progression is impaired during early prophase of meiosis I, followed by a marked and progressive loss of germ cells with age. Belongs to the argonaute family. Piwi subfamily.