Statistical mechanics predicts that the design of pure organized heteropolymetallic chains of metal ions bound to linear receptors depends on controlled deviations from the mixing rule DeltaE(MiMj) = 1/2 (DeltaE(MiMi) + DeltaE(MjMj)), whereby DeltaE(MiMj) is the intramolecular intermetallic interaction between neighboring metal i and metal j along the receptor. A thorough investigation of linear polymetallic trivalent lanthanide triple-stranded helicates shows that such deviations are amplified by an increase in the nuclearity of the final complexes and are thus easily evidenced in the tetranuclear heterobimetallic helicates [La(4-y)Lu(y)(L6)3](12+) (y = 0-4). The chemical and physical origins of this unprecedented behavior are discussed together with its practical consequences for programming pure heteropolymetallic 4f-4f complexes.