We report the results of photoluminescence measurements on ZnO bulk crystals implanted with both stable and radioactive species involving the group IV impurities Ge, Si and Sn. We previously confirmed the identity of a line emerging at 3.3225 eV as being related to Ge and present here uniaxial stress data which show that the defect responsible has trigonal symmetry. Experiments with Si provide circumstantial evidence of a connection with the well-known line at 3.333 eV. Our measurements indicate that for the case of Sn on the Zn site luminescence is not observed. We also confirm that the I9 and I2 lines are due to substitutional In impurities.