A rare case of low-percentage trivalent lanthanide doping in multinuclear lanthanide polyoxometalates (LnPOMs) was investigated. The [La4(MoO4)(H2O)16(Mo7O24)4](14-) polyanion was chosen as the host material for this study. In this polyanion the central [La4(MoO4)](10+) core is coordinated by four heptamolybdate groups as well as 16 water molecules. The tetranuclear lanthanum POM was doped with 5% of Eu(3+), Tb(3+), Sm(3+), Dy(3+), Nd(3+), Er(3+), and Yb(3+) (according to synthesis), and the structures and luminescence properties of the x%Ln:LaPOMs were investigated. Additionally a series of tetranuclear lanthanide POMs built from [Mo7O24](6-) heptamolybdate polyanions with Eu(3+), Tb(3+), Sm(3+), Dy(3+), and Nd(3+) instead of La(3+) were synthesized, and a detailed analysis revealed that the tetranuclear clusters formed monomers or dimers linked through oxygen bridges. The smaller lanthanide ions, namely, Er(3+) and Yb(3+), did not form tetranuclear clusters, but instead mononuclear sandwich-type POMs were obtained. The obtained structures were shown to be lanthanide-specific, and not a result of different synthetic/crystallization conditions. The luminescence properties of the x%Ln:LaPOMs were compared with the luminescence properties of the LnPOMs.