Just my 2 cents, but I sure wouldn't think so. If it is always P0304, indicating the misfire's always on #4, I wouldn't expect the issue to be only on #4 with CPS failure.
Since you say you changed out the coil packs with the other 5 cyls (#6 is different of course), I assume you swapped the plug wire with the coil pack so that shouldn't be the problem.
The only thing I can think of is possibly the #4 injector is not operating up to snuff (sealing O rings are weak/compromised, or injector slightly stopped up), or possibly a bad valve or a bad ring on #4. A compression test on numerous cyls with comparison should determine if that is issue. If it's running good, can't be too bad.
BTW, I'm not aware of any Autolite plug that Isuzu believes will work with the Ion Sensing ignition on this car. Veh will run fine on numerous plugs other than those recommended I know, but usually results in continuous P0300 random misfire codes that are impossible to totally eliminate. Since your misfire is always on #4, however, that's not likely your issue here.
Just not sure what sensor you are asking about on back of engine. Can you post a pic? You're not talking about the fuel pressure regulator are you? It's in the spot you describe, & operates on engine vac.