If you haven't already, disconnect the ECU and the ignition coil #4. Check the resistance between #4 signal wire and ground. It should be open.
I think the next step would be to temporarily run a wire from the ECU to the signal wire on the ignition coil.
Use the factory shop manual to find the location of the cylinder #4 ignition coil wire. On my 2011 G37 it is pin #11 and the wire is green. The 370 might be the same or different.
At the ECU tap into the #4 coil wire and run it to the signal wire on #4 ignition coil. Since this is just a test, make it easy for yourself and run the wire out the window to the engine.
** Make sure to use a wire that is about as thick as what is already there. **
The point of the test is to have a known good wire between the ECU and ignition coil #4 signal wire.
• Test for spark strength (like you did before) and compare it to #2 or #6
• Start the engine and check to see if it is running on all 6 cylinders.
I wish you luck and let me know the results.
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