The diode and resistor setup, with the diodes and resistors in parallel, you described is known as a "single fire adapter" and is usually used to adapt a dual fire tach to a Harley with 2 coils. I'm surprised at the results because the diode and resistor setup was sending twice as many pulses to the tach than when it was hooked to just one coil.
Because it fires once per revolution our shop tach has to be set in 2 cycle mode to get the correct rpm
's from a Road Star.
Does the tach have a switch for different numbers of cylinders? If it does then 4 cyl should work when hooked to one coil. The inductive test tach I have at home reads correctly when hooked to the plug wire in 4 cyl mode.