In view of a wide perspective of 3d-4f complexes in single-molecule magnetism, here we propose an explanation of the magnetic behavior of the two thiacalix[4]arene tetranuclear heterometallic complexes (Mn2Gd2III)-Gd-II and (Co2Eu2III)-Eu-II. The energy pattern of the (Mn2Gd2III)-Gd-II complex evaluated in the framework of the isotropic exchange model exhibits a rotational band of the low-lying spin excitations within which the Lande intervals are affected by the biquadratic spin-spin interactions. The nonmonotonic temperature dependence of the chi T product observed for the (Mn2Gd2III)-Gd-II complex is attributed to the competitive influence of the ferromagnetic Mn-Gd and antiferromagnetic Mn-Mn exchange interactions, the latter being stronger (J(Mn, Mn) = -1.6 cm(-1), J(s)(Mn, Gd) = 0.8 cm(-1), g = 1.97). The model for the (Co2Eu2III)-Eu-II complex includes uniaxial anisotropy of the seven-coordinate Co-II ions and an isotropic exchange interaction in the Co-2(II) pair, while the Eu-III ions are diamagnetic in their ground states. Best-fit, analysis of chi T versus T showed that the anisotropic contribution (arising from a large zero-field splitting in Co-II ions) dominates (weak exchange limit) in the (Co2Eu2III)-Eu-II complex (D = 20.5 cm(-1), J = 0.4 cm(-1), g(Co) = 2.22). This complex is concluded to exhibit an easy plane of magnetization (arising from the Co-II pair). It is shown that the low-lying part of the spectrum can be described by a highly anisotropic effective spin-1/2 Hamiltonian that is deduced for the Co-2(II) pair in the weak-exchange limit.