The magnetic properties of lanthanide (Ln) pnictides and chalcogenides have been compioled in subvolume 27B. Subvolumes 27B1, 27B2 and 27B3 (already published) deal with Ln monopnictides, monochalcogenides and binary polypnictides and polychalcogenides, respectively. Subvolume 27B4 is devoted to the ternary Ln pnictides. The first part 4a deals with ternary 1:1:1 and 1:1:2 type compounds, while a second part 4b will cover the types 1:2:2, 1:4:12, 3:3:4 and compounds with even more complex formulae.–Ternary Ln pnictides and chalcogenides are intensively studied due to fundamental importance resulting from the f-d interaction and hybridisation, and also due to problems and properties related to heavy fermions, non-Fermi liquid state, spin fluctuations, etc., and also due to the hope of applying these compounds as magnetic, thermoelectric or superconducting materials.