Phospholipid scramblase that promotes phosphatidylserine exposure on apoptotic cell surface. Phosphatidylserine is a specific marker only present at the surface of apoptotic cells and acts as a specific signal for engulfment. a 1,2-diacyl-sn-glycero-3-phospho-L-serine(in) = a 1,2-diacyl-sn-glycero-3-phospho-L-serine(out) Phospholipid scramblase activity is activated upon caspase cleavage to generate the XK-related protein 4, processed form. Does not act prior the onset of apoptosis. Homodimerizes upon caspase cleavage. Phospholipid scramblase activity is activated following interaction with the processed C-terminus of XRCC4 (protein XRCC4, C-terminus). Homodimer; homodimerization takes place upon caspase cleavage. Interacts with the processed C-terminus of XRCC4 (protein XRCC4, C-terminus); interaction promotes the phospholipid scramblase activity. Undergoes proteolytic processing by caspase-3 (CASP3), caspase-6 (CASP6) and caspase-7 (CASP7) to generate the XK-related protein 4, processed form, leading to its activation. Belongs to the XK family.