Contributions to the shift, width and vibronic splitting of a line in the vibronic sideband of an electronic transition are given to fourth order in electron-phonon coupling, including anharmonic and nonlinear interactions. The leading contributions represent the width and shift of the basic no-phonon line, including the non-additive elastic scattering contribution, together with the anharmonic width and shift of the phonon associated with the transition. Further contributions are found corresponding to interaction between the electronic and lattice states associated with the transition. These contributions essentially have a double effect on the observed width; they are important in the treatment of vibronic splitting. It is shown that the shape of an infrared phonon absorption line depends on whether the coupling with the photon proceeds directly or is mediated by the electronic system; in the latter case the spectrum is broadened by electronic interaction.