The origin of the excited species giving rise to radiative emission in ultra-thin films of quaterthiophene grown on potassium acid phthalate single crystal by organic molecular beam deposition is investigated. In the steady-state photoluminescence (PL) spectrum collected at 80 K two emission peaks at 2.52 eV and 2.43 eV with their vibronic progression, are detected. The PL emission at 2.52 eV decays on the time scale of 500 ps, while the lifetime of the second one is only 4 ps. These results allow to relate the two emissions with localised and delocalised states, respectively.