Old Man Rock wrote:
"If one end of the coil is disconnected, the damage can happen. The safest is to remove the power from the coil."
I get the concept of neither spark plug of a given coil pair not being grounded.... I was speaking in the regards if he had at least one plug #4 as in example still grounded then damage to this coil should have been minimized or elliminated due to the current path trhough the coil was still occuring...
OMR
Both plugs in the pair must have a path to each other. Normally this path is incidently going to ground, but it's not necessary. What really needs to happen is the current muct go from one plug wire to the other plug wire on the same coil.
The engine is used to complete this path, and yes the engine is grounded. It would work the same even if engine was not grounded. (Unlike in a classic car where the engine is part of the ground path to the coil.)
The sprk plugs for one coil are in series, not parallel, so both plugs are needed to complete the path.