Low voltage at coil and no spark

A pickup coil is typically 300 Ohms, but an Ohms test is inconclusive, because a shorted turn renders them broke. A turn short cannot be measured with VOM.

More important is to measure the (-) coil lead, and the supply side of the ballast. 4V seems low, since the coil primary resistance is about the same as the ballast, and the transistor that pulls the coil low may drop a volt there, the coil (+) should be about 7V with coil energized. Reading the supply side of ballast measures switched battery voltage. It might be low.

Also check reluctor to pickup gap with brass feeler. The gap is critical for starting, since the VR signal increases with RPM. That might be the reason it ran, but is unable to restart.
Three things can make the (-) coil lead always low (when it goes high spark happens). The VR signal is too low due to failure, poor 2 wire connection, or output transistor is shorted. The output transistor can be checked by disconnecting harness, and use VOM to measure resistance at transistor body to ground. Good is high resistance, bad is several Ohms.