The Cairo lattice is a tiling of irregular pentagons, each with 4 long and 1 short sides, thus producing two competing nearest neighbour interactions. When the short bond interaction dominates, a collinear structure is predicted by theory for low spins, whereas a possible spin nematic phase could be stabilised by a 4-spin exchange term when the two interactions are equal or the long bond interactions is larger. Bi4Fe5O13F is a physical realisation of the Cairo lattice, with pentagonal layers of Fe3+ (S=5/2) spins interleaved by a Fe3+/Bi3+ layer where the Fe3+ forms a square lattice, which is expected to be only weakly coupled to the pentagonal plane by an almost 90 degree Fe-O-Fe superexchange. Within the plane, the short interaction is expected to dominate as it involves superexchange via a 180 degrees Fe-O-Fe bond. As an initial determination of these interactions, we propose to measure the powder magnon spectrum of Bi4Fe5O13F on IN4C. Spin wave calculations show that much information may still be extracted from a powder measurement if guided by ab initio calculations, which have been completed.