Lane B Engine ECU Nominal Current not reached.

How would you suggest the plug be "cleaned"?   I'm only suspicious of the Lane B plug, but could there be other plugs that have cylinder 3 and 4 in common that I could investigate?   
The ECU plugs are probably the most isolated plugs. 

One thing I didn't mention is that this fault occurred after being out in the weather one night with some light rain.  It didn't fault until near the end of the 1 hour flight near the FAF in IMC.

Your fault message suggests an ignition fault on cylinders 3 & 4.  Coil SP-3 fires the bottom plugs of cylinders 3 & 4 and is controlled by Lane A.  Coil SP-4 fires the top plugs of these same cylinders and is controlled by Lane B.  There is a good diagram of this in section 74-20-00, page 5 of the HMM.   Try swapping coils SP-3 and SP-4 and see if the problem moves to lane A.  If it does coil SP-4 is bad.  

12V+  is supplied to each coil from the fusebox and is energized whenever the fusebox is powered.  Each has its own circuit fused at 10 amps.  The ECU controls the 12V- to the coils, switched as required for ignition.  So you can see that this could also be a fusebox connection problem at connector X1 pin 16, or connector X2 pin 15, or the associated fuse holders themselves.  

I believe I found the root cause.  I pulled coil #4 that services the 3T and 4T plugs, and cleaned the contacts, applied some dielectric grease to prevent oxidation, then ran the engine.  No faults.  Flew several circuits without indication of a fault.  After the flight I pulled the contacts on all of the coils and cleaned and applied dielectric grease to each.  All of the other coil's connectors were much tighter than the ones on coil 4.  Since this adjustment, I have now completed 5 flights without any fault.  I consider this successfully root caused and resolved.  I fed back this info to the engine guy at the factory in Torrance.  He said that there have been some issues seen on these contacts.

I've been finding faults caused by different electrical connections wiggling from vibration, even the ones at the ECU. I now remove connectors when someone complains of faults and apply dielectric grease and then push them back on firmly. So far it has been working. Bad connection blips are one reason Rotax uses the metal clips on the two outside probes on the airbox and we wire tie the two center ones. It just takes a vibrating wiggle of a connector to cause a fault even though it really isn't a true fault.