A photoionization code has been developed incorporating provision for the presence of dust mixed with gas. M4-18 has been modelled utilizing optical CCD spectra and other observations ranging from UV to radio wavelengths. T_e and N_e from the model are 6600K and 6050 cm^-3, respectively. The abundances of C, N and Ne are found to be the same as the solar values while O is overabundant by a factor of two and S is lower than the solar value by ~90 per cent. A mono-sized amorphous carbon grain dust model explains the IRAS fluxes satisfactorily but emits poorly in the 1- to 10-μm region. M4-18 is a protoplanetary nebula with properties similar to CPD - 56 deg8032 and V348 Sgr.