The effects of the lithium substitution for copper on the properties of the superconducting YBa2Cu4O8 were studied. Single-phase YBa2Cu4-xLixO8 was successfully prepared by the sol-gel method under ambient pressure over a composition range of 0 less than or equal to x less than or equal to 0.08, while impurity phases appeared at x=0.10 and 0.20. The lattice constants of YBa2Cu4-xLixO8 were almost invariant with increasing lithium content x. The superconducting transition temperature decreased monotonically with increasing x in the range, 0 less than or equal to x less than or equal to 0.08. The suppression of superconductivity was discussed in terms of the impurity effect in the CuO2 planes.