What everyone needs to keep in mind, just because you have a trouble code for a single cylinder misfire does not always mean you have a problem with only the cylinder that triggers the code.
Depending on how quickly you read the code and possibly clear it, you may only see the single cylinder error, but if you do not clear the codes and continue to drive, you may pick up additional cylinder misfire codes.
Also many times a single cylinder misfire could be an indicator of a cylinder with parameters that are further out of spec than others, or more susceptible to a misfire due to intake mixture, spark plug condition, low compression, excessive carbon build up, injector spray pattern and volume and other issues.
Sometimes you get lucky and there is a "hard" failure that is easy to find, however, many times you are unlucky and you are searching for a needle in a haystack for a specific solution.
For example, a DISA with a blown vacuum actuator, will cause a stream of unmetered air to be introduced somewhere into the rear 3 cylinder area of the intake. Many times cylinder #4 may be mostly impacted, however, cylinder #5 could be impacted as well.
Additionally there are a number of vacuum connections on the rear underside of the intake that can impact the rear cylinders. I do believe there is one port near the #5 intake port?
If you look at the MOST common causes of misfires and fuel mixture problems, you will see the overwhelming majority of the problems are due to unmetered air leaks/vacuum leaks, usually due to dry rotted and damaged hoses and/or gaskets that have hardened in a compressed state and fail to expand back to full size once an engine cools down.
Yes there are problems with coils, spark plugs, injectors, valve train, cylinder compression, camshaft lobes, wiring, connectors and so on, but usually these "hard" part failures are not as intermittent and are usually much easier to positively identify.
But the point I continue to drive home is sort out the simple, cheap and obvious problems first before pulling cylinder heads or replacing DME/ECU and so forth.
Rubber parts are reliable on average for about 7-8 years before they deteriorate quickly. All these cars are getting just to the age of the witching hour and it is time for preventive maintenance.