We investigate the conversion between transverse mode structures in four-wave mixing in a heated rubidium vapour. While angular momentum conservation in this nonlinear process dictates the selection rules for the angular quantum number, the role of the radial quantum number is more esoteric. We demonstrate experimentally that a clean Laguerre-Gauss mode LG p =LG 0 1 can be generated by converting LG 1 0 and LG −1 0 near-infrared pump beams -but only if the length of the atomic medium exceeds the Rayleigh range. In the same regime we also observe a direct transfer of the total radial quantum number from input to generated light modes.