No signal to Ignition Coil 4

Thanks David.  You are correct and I stand corrected:-ignition coil 4 does fire 3T and 4T spark plugs.  During the replacement of Coil 4 with a new one (which didn't fix the problem) after having unscrewed the two high tension leads from the coil I checked resistance with one probe of multimeter at end coil end of high tension lead and other probe of multimeter at spark plug cap and both were the same at around 18 ohms.

I'm convinced the problem is 'upstream' of the coil.   Will next check the four fuses you mentioned.

I’m hoping to hear from Rotax soon if the found a fault in the returned parts from my engine. Our problems sound very similar. I’ll post here if I get a cause from them. 

Another thing you can check is the continuity from the fuse to the spade connector. I think on my engine I was getting about 0.3 ohms. Eventually Rotax also had me check the wires from the ECU to the coils. Those are the connectors with the 20 cycle life limit on them. Rotax has a test connector to attach so you don’t damage anything with your meter probes. In my case all these wires showed good continuity. 

And I agree with you that he problem is likely upstream from the coil. 

Have checked fuses F6-F9.  All visibly intact.  For avoidance of doubt, all four were replaced with new ones and that didn't make any difference.

To my thinking the history of the problem wasn't consistent with a blown fuse because of the progression and development of another symptom after the first couple of hours of solid Lane B light: - 

During the first couple of hours of solid Lane B light, if Lane A switch was turned off (my understanding is Lane B MUST take over engine management and spark control if Lane A is switched off) the engine still ran smoothly, indicating at that time spark was still being generated by Coil 4 which is controlled by Lane B, and that spark was reaching both cylinders fired by Coil 4.

However, since then, if Lane A is turned off, the engine now runs extremely roughly (as though only firing on 2 or 3 cylinders) and fuel pressure rises, indicating that no spark at all is being generated by Coil 4 and at least one fuel injector is not injectingl?

I had the same. In my case Lane A had the fault. Fault light would go out when master and lanes turned on. After engine start Lane A would show fault with in a few seconds to a few minutes. With Lane A fault on Lane B could be switched off and engine continued to run on the bad lane. It did run rough in this condition. 

I wasn’t thinking of a blown fuse. Maybe a weak connection or electronic component in the fuse box. it was curious that power cycling the airplane always reset the lane. 

As I mentioned before I hope to hear from Rotax soon if they can tell me the component at fault. With all the replaced parts the engine now continues to run well