Injectors went for a retest, and guess what, they failed the brand new injector for having no back leakage, not the kind of thing to cause a misfire apparently.
This point has been proved tonight by refitting all injectors with the new one again in cyl 4, recoded and start up, runs ok for 1 min then cuts out, no cough or splutter just immediate stop. Restarts with a distinct misfire, reads code and no 4 injector is mentioned again, so it seems as Kev and Mark have pointed out, that I will be removing a wiring loom over the weekend if it will stop raining long enough, just need to have a word with the German who designed the tamper proof cap for the Ecu plugs, will have to see if there is a blade in the angle grinder.
Did a brief multimeter check between the Ecu as close to the plug as I can get, and the injector plug, found no fault good continuity and no crosstalk between the wires, but as I said was only a brief check with time and light running out. Being as this my own van I think I will probably end up replacing the loom for a new one, despite how much it galls me to shell out on a van of this age, but considering we have also seen a no1 injector code a repair would seem to be futile. Once again I will update you when I have more news, and spent more money at Mertrux.