Generates 3->3 cross-links in peptidoglycan, catalyzing the cleavage of the mDap(3)-D-Ala(4) bond of a tetrapeptide donor stem and the formation of a bond between the carbonyl of mDap(3) of the donor stem and the side chain of mDap(3) of the acceptor stem. Is specific for donor substrates containing a stem tetrapeptide since it cannot use pentapeptide stems. Is irreversibly inactivated by the beta-lactams carbapenems via the formation of a covalent adduct resulting from enzyme acylation; imipenem is the most efficient drug for in vitro LdtMt4 inactivation. Cell wall biogenesis; peptidoglycan biosynthesis.