[Mark44]

Looks like the problem has been solved - fingers crossed. As already mentioned, the cylinder 4 misfire (P0304) has been plaguing me for the two years I've owned the car, and was noted on two invoices for services for the previous owner.
After getting no joy after replacing the injectors and the coils, I took it into a shop that specializes in repairs for German cars. They used a borescope to determine that one of the intake valves wasn't opening all the way, which points to a malfunctioning intake lifter. They replaced both intake lifters on cyl. 4, and so far, the engine is back to running how it should.

The lifters aren't all that expensive, at about $150 for the pair, but the labor involved ran to 13 hours (engine was not removed).

In case you're not aware, the intake lifters on this vehicle are pretty trick, being operated hydraulically and electrically. The lifter is cylindrical, with an inner cylinder inside an outer cylindrical shell. Each intake valve is paired with three cam lobes, an inner lobe and two outer lobes. As I understand it, at low engine speeds, the inner core of the lifter rides on the inner camshaft lobe. Under higher engine loads, the inner lifter retracts into the lifter and is locked in place by a solenoid, and the lifter then rides on the two outer cam lobes, which cause higher valve lift and longer duration. The Bentley manual has pictures of the lifter and cam lobes.