Involved in DNA non-homologous end joining (NHEJ); required for double-strand break repair and V(D)J recombination. Binds to DNA and to DNA ligase IV (lig4). The lig4-xrcc4 complex is responsible for the NHEJ ligation step, and xrcc4 enhances the joining activity of lig4. Binding of the lig4-xrcc4 complex to DNA ends is dependent on the assembly of the DNA-dependent protein kinase complex DNA-PK to these DNA ends (By similarity). Belongs to the XRCC4-XLF family. XRCC4 subfamily.