"Event button print-out with continuous ignition switched ON showed only two of the four ignitors operating."
Most likely, Cornish, the EIU's will only know what positions the ignition control relays (which control the ignitor boxes and, ultimately, the ignitors) and cockpit switch(es) are in. If you have a copy of the Boeing Schematics, for example Chapter 74-31-0x p101 Sheet2 (where x is the engine), this will show some of the ignition inputs to the EIU's. In this diagram you will see that an additional contact on the ignition relay is used to send an ignition status signal to the EIU. As you suspected, the EIU doesn't really know if the ignitors are actually working or not (and an audible check is required)... It's only "guessing" that the other relay contacts and the ignitor boxes are operating normally (i.e until the engine flames out(grin).
The relays are either full on or full off, so there is no cycling of relays which would cause your strange(?) Event recording. I suspect it is an EIU software thing rather than a malfunction of ignition ccts(Sorry, I don't know if this is normal for all 747-400 airlines/engine configs).
But... I hope this helps.
Cheers.
Q.