No signal to Ignition Coil 4

Sounds like you've ruled it out, but I've had chaffing on a couple ignition wires.  I dont recall which plug/cylinder it was for, but the worst one was pilot-side, high tension wire comes out of the coil, just after the 90 degree bend, it was rubbing on the rough surface of the intake manifold.  Only took about 400 hours to eat through the insulation.

Check the firewall-side fusebox.  There are four 10A fuses for ignition in there.  F6, F7, F8, F9.  It looks like Coil 4 supplies spark to #3 top and #4 bottom..  I assume you don't see spark on either.

I’ve just got things back working after having a similar problem on the Lane A side. I was getting a lane A fault light with the message “CYL 1 + 2 Ignition”. Replaced coil leads and spark plugs and swapped ECUs. Checked continuity from the fuse box and ECU. Nothing fixed it. Finally changed the main engine wire harness and fuse box. Problem is now fixed. I’m waiting now to hear from Rotax where the problem was when they check the returned parts. Guessing it was a fault in the fusebox, loose connection or bad wire.Fortunately my engine is still under warranty. 

Thanks very much Robert and David.

No spark at either of the plugs fired by Coil 4 (which are #3 top and #4 bottom) when engine turned over in the dark.

Will check fuses .

If your engine is an iS the wiring diagram shows coil 4 fires the top plugs for cylinder 3 & 4. They refer to the coil  as having a top and a bottom wires but this refers to the position the wires exit the coil. Follow them on the diagram and you’ll see the wires both go to top plugs. 

You can also check resistance on plug pairs through the coils. The total resistance should be 18 ohms from spark plug cap to plug cap. You’ll be able to see which plugs are paired this way too.