We study the possibility of introducing programmable filters in the Fourier plane of an optical free-space wavelength demultiplexer to provide wavelength-band selection or programmable optical add and drop multiplexing. Various 4f configurations are modeled, and we compare the respective advantage of two main cases: the first where the dispersive element is superimposed in the Fourier plane with a programmable transmittance, the second where the dispersive element is combined with the lens(es) and the Fourier plane is free to set the programmable element. An analytical model of the beam propagation is proposed in the case of a Gaussian beam to make easier the choice of adjustable parameters and optical configurations. The potential use of the Fourier plane free configurations is assessed in terms of wavelength-band resolution, selectivity, and attenuation dynamic range.