The Cu PL defect is characterized by intense photoluminescence (PL) emission with a 1014 meV zero-phonon line and has since long been called the 'copper-pair' defect. However, recent PL studies in isotopically pure 28Si samples show clear evidence for four Cu atoms in this defect. This defect is one of a family of complexes that contain four (sometimes five) metal impurities. No complex containing two or three metal impurities (at least one of which is a transition metal) has been reported. We have performed systematic calculations of a priori possible Cu 4 complexes, including the Cu s1Cu i3 complex proposed by Shirai et al. . This complex has the lowest formation energy of all the Cu 4 defects. We have studied its properties using first-principle theory. In addition to the structures, binding energies, and vibrational spectra, we studied the formation pathway in an attempt to figure out why smaller (or larger) complexes do not form. We find that Cu s1Cu in with n=0, 1 or 2 continue to trap Cu i if the Fermi level is midgap, and that the process stops when n=3.