BMW Repair: 1996 740il cylinder misfire, mass air sensor, valve cover gasket

Question
Check engine light went off giving me 3 fault codes on my Peake FCX II.It says fuel injector(unknown),ignition coil cyl#4, and another code but the engine is running so smooth, no hesitation on acceleration. Since its been over 6 mos since the spark plugs have been changed I decided to change all 8 spark plugs specifically for this model. I used identical bosch plugs but afterwards, the check engine lights gave more fault codes and this time the timing is off and clearly few cylinders were misfiring coz its shaking and lacking power.I did not check the gaps of the spark plugs because I was told they are pre-gap by bosch. I have read likewise that the OBD fault codes for BMW is not reliable or specific. I have brought it before to Shelly BMW dealer and had me change $400 mass air sensor but did not seem to show any improvement nor resolve the fault codes showing. I love driving BMW but starting to have mix feeling about the car because of difficulty and exhorbitant cost of repairing it. Don't mind spending but not when there's always something goin wrong and no one really is sure how to fix it. One mechanic wants $420 just to look at it saying it takes minimum 4 hours to check/replace the plugs. I've seen it done over 1 hour. I'd appreciate any advise you can extend or if you know anybody that's honest and reasonable around Los Angeles. Thanks, looking forward to hearing from you.  

Answer
when the spark plugs were replaced, was there oil in the recessed area where the spark plug goes? if so, this can cause a misfire. the oil gets in there from a leaking valve cover gasket. get back to me and let me know. 4 hours to replace plugs?! an inspection 2 service takes 4 hours and it includes oil/filter change, spark plug, fuel filter, reading out fault codes, checking tires, brakes, and overall vehicle. they are trying to rip you off! let me know what the new fault codes were as well.