We have studied the effects of substituting gallium by aluminum, germanium, tin and indium in PuCoGa5, the actinide-based superconductor with the highest critical temperature. By annealing arc-melted samples, we have been able to synthesize only compounds with 20% substitution by Al and Ge (i.e. PuCoGa4Al and PuCoGa4Ge). Refinements of X-ray powder diffraction patterns for these two compounds indicate an enhanced c/a ratio, whereas magnetic susceptibility measurements reveal a large reduction of the critical temperature Tc compared to PuCoGa5. DFT+ED calculations indicate that the decrease of Tc is accompanied by significant changes in the Fermi surface. Finally, measurements on PuCoGa5 samples made by using 69Ga and 71Ga isotopes showed no effect of the atomic mass of gallium on the superconducting critical temperature.