To check this better I am considering the propane test, adding propane to the inlet manifold to richen the mixture. If the fuel trim is driven rich on bank 4 then this should more conclusively point to an injector issue (surely). If not and bank 4 still reports a lean fuel trim then it should be that defective 02 sensor.. any thoughts?
Thanks,
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Present cars
Daily driver: BMW e65 730d
Batmobile: 2006 S8, D3, 4E 5.2L V10 BSM with (loud) Miltek SS exhaust, Carbon Fibre trim, Alcantara headlining, soft close and keyless entry. 
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