To try and sum up...

SPARK - You've changed the plugs and coil packs. So IF the plugs are correct 41-108, and in good condition, then it's not likely to be spark.

MECHANICAL - I had poor compression in #4 cylinder(coked valves and cylinder/ring wear), and did NOT get any misfire(or any other) codes. I like your compression numbers, I don't think this is coked valves or other internal mechanical problem. Grasping here, could a valve stuck closed cause this(anyone)? Well, on all 4 cylinders, and intermittent, extremely unlikely.

AIR - I suppose a decent-sized vacuum/boost leak could be the culprit. You would very likely have other codes. How much vacuum do you get at idle, engine warmed up? How much peak boost do you get?

FUEL - You said this is your wife's ride so maybe you don't know the answer to this question - Do the codes set only(or mostly) under heavy load? Maybe try babying it, see if it still sets or not.

You haven't answered DrLoch's question - how were the injectors cleaned?

There are conditions where high side fuel pressure required is over 2,000 psi. I am leaning towards a fuel problem. High pressure fuel pump and/or injectors.

 

Vacuum at idle = 20
Max Boost = 15

I am not certain of the procedure dealer used to clean injectors.

 

Have you looked at all of the connectors especially the one near the A/C. The wire bundle that butts up against the EVAP purge valve bracket. Any other wire that might change it's orientation under vibration.

 

The dealer just ran a "special fluid" IE store bought injector cleaner through your car. They just let it idle after hooking it up to there machine that I think goes into one of the vacuum lines.

 

That would be zero benefit to the injectors. Remember the 2.0 is DI, so the injectors aren't anywhere near vacuum lines.

The best way to clean them is like DrLoch described. This was probably too much labor, they probably just dumped chemical in the fuel tank and ran it for awhile. This is not very effective, but might clean them very slightly, if cheap fuel has been the norm.

 

They also claim that the service cleans the throttle body as well. It's one of those oversell services a "fuel system cleaning".

 

That cleaner in the fuel tank or in the vacuum line would not get near the throttle body .

 

Chevy HHR SS Random Misfire update.

The car mysteriously cleared the misfire codes and ran fine as a daily driver for about 4 weeks. I couldn't correlate the misfire clears to any recent changes to the car since I had been out of town.
Yesterday, when the wife was pulling out of a parking lot. Low rpm, near idle, she said she heard a "swoosh" sound and then noticed that check engine light came on steady at the same time. Car did not appear to hesitate as she pulled out of the lot.
Checked the codes today when I got home: P0300, P0301,P0302,P0303 and P0304 which are the same codes which we were getting before. Anyone have an idea what this "swoosh" sound may be? or how it may be related to the misfires?

 

Impossible to be sure what the swoosh sound was without being in the vehicle at the time. I might guess that it was a bad stumble from the misfires.

 

Check clamps on the charge pipe, or cracks in the charge pipe