A novel double-armed calix[4]arene derivative, 5,11,17,23-tetra-tert-butyl-25,27-bis{2-[N-(2-hydroxy-3-methoxybenzylidene)amino]ethoxy}-26,28-dihydroxycalix[4]arene (3), has been synthesized by the reaction of 1,3-alternately lower-rim-modified bis(2-aminoethoxy)-substituted calix[4]arene (2) with o-vanillin in 90% yield. Calorimetric titrations have been performed in anhydrous acetonitrile at 25 °C to give the complex stability constant (K_S) and thermodynamic quantities for the complexation of light lanthanoid(III) nitrates (La–Gd) with 3. Using the present and reported data, the complexation behavior of the structurally related cyanomethyl analogue (1), aminoethyl analogue 2, and 3 is discussed comparatively from a thermodynamic point of view. Possessing donating vanillin side-arms and structural flexibility, 3 gave the highest K_S for Eu³⁺ among the light lanthanoid series. Having the same calix[4]arene skeleton in common, these hosts afforded distinctly different cation selectivity profiles. Thus, fairly low and invariant K_S values over the entire light lanthanoid series were obtained with the cyano analogue 1, while the incorporation of amino side-arms in 2 greatly enhanced the K_S for La³⁺ and less extensively for Ce³⁺ but no change was observed thereafter, giving a sharply declining pattern from La³⁺ to Pr³⁺ followed by a sudden leveling-off. In contrast, the vanillino analogue 3 showed a global enhancement of K_S for all light lanthanoids, displaying a unique selectivity profile with a broad peak at Nd³⁺ and a sharp spike at Eu³⁺. These results are accounted for in terms of the three-dimensional induced fit to lanthanoid ions by the lower-rim and side-arm donor atoms, which play a decisive role upon recognition of trivalent lanthanoid ions by the calix[4]arenes with flexible donating side-arms. Thermodynamically, the complexation is absolutely driven by the favorable enthalpy changes, but the cation selectivity is mainly determined by the less negative entropy changes.