[DNA repair protein XRCC4]: DNA non-homologous end joining (NHEJ) core factor, required for double-strand break repair and V(D)J recombination. Acts as a scaffold protein that regulates recruitment of other proteins to DNA double-strand breaks (DSBs). Associates with NHEJ1/XLF to form alternating helical filaments that bridge DNA and act like a bandage, holding together the broken DNA until it is repaired . The XRCC4-NHEJ1/XLF subcomplex binds to the DNA fragments of a DSB in a highly diffusive manner and robustly bridges two independent DNA molecules, holding the broken DNA fragments in close proximity to one other. The mobility of the bridges ensures that the ends remain accessible for further processing by other repair factors. Plays a key role in the NHEJ ligation step of the broken DNA during DSB repair via direct interaction with DNA ligase IV (LIG4): the LIG4-XRCC4 subcomplex reseals the DNA breaks after the gap filling is completed . XRCC4 stabilizes LIG4, regulates its subcellular localization and enhances LIG4's joining activity. Binding of the LIG4-XRCC4 subcomplex to DNA ends is dependent on the assembly of the DNA-dependent protein kinase complex DNA-PK to these DNA ends. Promotes displacement of PNKP from processed strand break termini.; [Protein XRCC4, C-terminus]: Acts as an activator of the phospholipid scramblase activity of XKR4. This form, which is generated upon caspase-3 (CASP3) cleavage, translocates into the cytoplasm and interacts with XKR4, thereby promoting phosphatidylserine scramblase activity of XKR4 and leading to phosphatidylserine exposure on apoptotic cell surface.