A sandwich-type polyoxometalate, [Cu-4(GeW9O34)(2)](12-) (1a), in which two B-alpha-[GeW9O34](12-) ligands sandwich a rhomblike Cu-4(8+) tetragon through alpha-Keggin linkage, is first isolated as a [n-BuNH3](+) salt, [n-BuNH3](12)-[Cu-4(GeW9O34)(2)] center dot 14H(2)O (1). A Cu4O14 cluster for the rhomblike Cu-4(8+) tetragon in la with C-2h local symmetry consists of two Jahn-Teller (JT) distorted CuO6 octahedra (at internal sites) with a short diagonal Cu-int center dot center dot center dot Cu-int distance of 3.10-3.11 angstrom and two CuO5 square pyramids (at external site) with a long diagonal Cu-ext center dot center dot center dot Cu-ext distance of 5.34-5.35 angstrom, the feature of which is different from [Cu-4(H2O)(2)(GeW9O34)(2)](12-) (2a), comprising the four JT-distorted CuO6 octahedral Cu-4(8+) tetragons through beta-Keggin linkage: the axial Cu-ext-O bond distance (2.27-2.29 A) for 1 a is shorter than the corresponding JT-axial distance (2.36 angstrom) for 2a. Measurements of magnetic susceptibility, magnetization, and electron spin resonance spectroscopy for 1 are carried out for better understanding of the molecular magnetism of the Cu-4(8+) tetragon in comparison with 2a. The analysis of the magnetic behavior, based on the isotropic Heisenberg spin Hamiltonian comprising three exchange parameters (J, J', and J '') gives J = -24.1 cm(-1) for the Cu-ext center dot center dot center dot Cu-int sides, J' = -99.1 cm(-1) for the Cu-int center dot center dot center dot Cu-int diagonal, and J '' = +0.04 cm(-1) for the Cu-ext center dot center dot center dot Cu-ext diagonal of the Cu-4(8+) rhombus. The S = 1 ground state of 1 displays g(parallel to) = 2.42, g(perpendicular to) = 2.07, D = -1.44 x 10(-2) cm(-1), and vertical bar A(Cu parallel to)vertical bar = 46.5 x 10(-4) cm(-1). An observation of the asymmetric magnetization between a positive and a negative pulsed field (up to 10(3) T/s) at 0.5 K on the hysteresis loop indicates the quantum tunneling at zero field. The magnetic exchange interactions of four unpaired d(x2-y2)-electron spins are discussed in terms of the point-dipole approximation, and the primary contribution to D is implied to come from the magnetic dipole-dipole interaction between two spins at the Cu-ext centers.