Meudon models of the chlorine chemistry in the two PDR layers seen towards FIR 4, showing abundances relative to n_gas. Left panel: dense PDR in the outermost envelope of FIR 4, with n_gas= 6 × 10⁵ cm^-3, G₀ = 415 ISRF. The cosmic ray ionization rate is ζ = 10^-16 s^-1 (solid lines) and ζ = 10^-14 s^-1 (dotted). The position is given outside-in from the right. Right panel: tenuous PDR in the foreground of FIR 4, with n_gas= 10² cm^-3, G₀ = 1500 ISRF and ζ = 3 × 10^-16 s^-1. The position is given from the strongly irradiated side on the right.