They are probably a 6v bulb and use about .25 amp each. So, you'd need at least an amp to drive 4 of them. At higher voltages, they'll run hotter (not good for the plastics) and probably won't last very long.
One possibility would be to get some decent low-voltage LED replacements and run your separate supply. You probably won't need as much current (check into the specs for the parts), they'll likely run cooler, and will work with the smaller "wall wart" type supply. Kludgey, but it would get the job done.