ECU is reporting injector electrical faults so that is where I would be looking. It does not need a camshaft signal to validate the state of each injector driver so the fault code is likely to be correct.
Check the connectors are pushed onto the injectors properly and that the locking clips are there (not some lazy bugger left them off previously) Look carefully at both wires to each injector, has one snapped?
Check the earth wires that attach to the fuel rail, are any broken, loose or corroded.
Follow the injector loom back, looking for any connectors, i think the 4 pot goes direct from the injector to the big engine loom connector by the battery so should be OK but I have seen broken / corroded wire on the V6s big injector plug & socket.
If the fault code has moved from #4 to #3 that suggests a bad earth or loose connector.