A Z engine will run on 5 cylinders. #4 not firing didn't cause the stall. At this point, not enough info to diagnose that. Compression ratio figures, I don't remember if this engine is new or old but either engine would run fine with these #s. If the engine is new, the #s should be very close if not the same. However, the valves MUST be adjusted to specs before ANY troubleshooting including a compression test. If a mechanic has been doing this in the past, IMO, YOU should pull the valve cover & check the clearances. It's the only way to be sure & it's not difficult. #4-compression is good, & it is fed by the same fuel supply as #5 & #6. The only reason the cylinder is not firing, IMO, is little to no spark at the plug. The coil supplies elec. to all the plugs through the distributer. The pathway to the plugs is very simple. As the dist. turns, the rotor distributes the elec. to the contacts inside the cap. From there it goes into the plug wires & into the plugs. Fact, there are 5 cylinders firing. The problem has to be located somewhere from the dist. cap to the plug. A quick check with an ohm meter will tell you if have a bad plug wire. Compare it to the others. Other options already mentioned, timing light on #4 plug wire or just trade #4 wire with another one to see if the problem moves with the wire. You've replaced the plug, if the wire is good the only thing left is the cap. Rare, but I recently had one go bad on one cyl. Ohm meter check, inside to outside & double check the connections at the cap & plug.