We propose to measure the high energy magnon spectrum of Bi2Fe4O9 in order to finalize a spin wave model and to classify this compound within the theoretical phase diagram of the Cairo pentagon lattice. Previous triple-axis spectroscopy (TAS) measurements showed only one excitation band whereas at least two are expected. This powder measurement will determine the energy and bandwidth of this higher band and thus determine the remaining exchange parameters which are not yet constrained by the TAS data.