We present the results of a combined magnetization, muon-spin rotation, transport and magnetotransport study of the n = 3 Ruddlesden - Popper (RP) compound . This compound adopts a layered structure in which groups of three perovskite layers alternate with single rock-salt layers. The muon-spin rotation data show that there is a sharp magnetic phase transition at 115 K. The resistance and magnetoresistance of the sample show no particular features at this temperature, but the transition affects the energy barriers associated with hopping transport. The magnetoresistance is proportional to the square of the magnetization, and is largest at low temperatures; a 40% drop in resistivity is observed in a magnetic field of 14 T at 61 K, much smaller than that measured in the related RP (perovskite) manganites which exhibit colossal magnetoresistance (CMR).