In the course of our exploratory synthesis of new quaternary (poly)bismuthide Zintl phases with large unit cell dimensions and potentially ultra-low thermal conductivity, we embarked on testing the alio-valent substitutions as an intuitive method to rationally prepare such materials, starting from ternary phases. We are reporting herein on the synthesis, crystal and electronic structure of the new structurally complex quaternary bismuthide Zintl phase Ba8Zn2-xIn3+xBi10 (experimental: x = 0.21-0.47) which is the first structurally characterized quaternary bismuthide Zintl phase. It crystallizes in the monoclinic space group C2/m with a large unit cell volume as anticipated, with two distinct one-dimensional (1-D) polyanionic units in the same crystal structure. The coordination site preference between Zn and In, which is dictated by difference in atomic properties like atomic size and valence state, appeared to be critical for the structural complexity and the significant phase width. Theoretical studies based on density functional theory (DFT) calculations were conducted to identify the main structure directing forces.