  An integrable quantum spin ladder based on the SU(4) symmetry algebra with
boundary defects is studied in the framework of boundary integrability. Five
nontrivial solutions of the reflection equations lead to different boundary
impurities. In each case the energy spectrum is determined using the quantum
inverse scattering method. The thermodynamic properties are investigated by
means of the thermodynamic Bethe ansatz. In particular, the susceptibility and
the magnetization of the model in the vicinity of the critical points are
derived along with differing magnetic properites for antiferromagnetic and
ferromagnetic impurity couplings at the edges. The results are applicable to
the strong coupling ladder compounds, such as Cu_2(C_5 H_12 N_2)_2 Cl_4.
