May be involved in DNA non-homologous end joining (NHEJ) required for double-strand break repair. May bind to DNA. The LIG4-XRCC4 complex is probably responsible for the NHEJ ligation step, and XRCC4 may enhance the joining activity of LIG4 (By similarity). Interacts with LIG4 and specifically binds its tandem BRCT domains (PubMed:11029705). Interacts with POLL (PubMed:23660835). Slightly induced by gamma radiation, but not by white light or by UV-B. Belongs to the XRCC4-XLF family. XRCC4 subfamily.