High-temperature superconductors YBa2Cu4O8 and Y0.9Ca0.1Ba2Cu4O8 and their composites with varied quantities of Ag2O were prepared using a modified solid-state reaction method. The critical transport current density, J(c), was measured extensively at temperatures below the superconducting transition temperature. A remarkable increase (greater than one order of magnitude) in J(c) was obtained for both superconductors by optimum silver addition. Reduction of the characteristic porosity in the 124 superconductors was found to be the origin of this unusual J(c) enhancement.