Check for codes, that should be first.
I might also inspect the wiring on the drivers side, coils and injectors.
Losing all four cylinders would make the car run incredibly rough and it would have no power. Losing one or two cylinders would give a substantial power loss but it would probably be fairly smooth still.
Do you encounter exhaust smell when it happens? Possibly indicating no spark but fuel still?
We need to figure out which system is causing the power loss first, fuel or ignition. Hopefully the OBD has a clue to start with.
Current ride... '13 GTI Wolfsburg
BMW's from my past...
'92 535i/5 '89 325ix/5
'98 750iL '86 325(e)/5
'95 740iL '84 318i/5