The new copper oxide orthoarsenate, Cu-4(AsO4)(2)O, was obtained from a mixture CuO and As2O5 . xH(2)O in a BaCO3 heated to 1090 degrees C. The material contains two copper ions in distorted trigonal bipyramidal CuO5 geometries with a third copper ion in a distorted square pyramidal CuO5 geometry. The free "oxide" ion is surrounded by four copper ions in a distorted tetrahedral arrangement. The copper ions form. extended two dimensional layers via a network of Cu-O-Cu bridges. The layers are interconnected via Cu-O-As-O-Cu bridging units. Magnetic susceptibility measurements show significant antiferromagnetic coupling of the unpaired electrons and an unusual transition into a weak ferromagnetic state in the temperature range 15-25 K.