Ceramic samples of the superconductor RBa2Cu4O8, with R=Y, Er, Dy, Gd, and Sm have been prepared and studied by Raman spectroscopy in the range of 180-700 cm-1. Most of the modes are found to shift more slowly with ionic radius than in the related compound RBa2Cu3O7-δ, although the expansion of the lattice is similar in the two materials. The one exception is a 340-cm-1 mode associated with the oxygen ions on the CuO2 planes, which has a mode Grüneisen parameter of 3.3 at room temperature. Raman spectra taken below room temperature show little change other than the narrowing expected from the increased lifetime as anharmonic two-phonon decay becomes less important. The 340-cm-1 mode is the exception; its frequency displays a temperature dependence that is especially strong in the sample with the largest rare-earth ion (Sm). Even in this case there is no change that can be related to the superconducting transition, in contrast with observations in RBa2Cu3O7-δ. The absence of changes near Tc is discussed in terms of the magnitude of the gap and its dependence on the concentration of holes on the CuO2 planes. © 1993 The American Physical Society.