jumped wires at ignition switch and got mower to run. Then my x bought the wrong ignition switch by accident, hooked it up, replaced the battery and now the magneto is shorted out. Then put the old ignition switch back on and get 4 volts d.c. to the magnito. Is this the normal?

How are you checking for voltage to mag?

DC Voltage should be the same as battery, minus a little drop for resistance in wiring. Is there a start resistor in the circuit?

Hi,
There should be no voltage at the magneto(ignition) as it will destroy the armature.
Peace,
Clarke