I now have most of my answer and, unfortunately, it's not what I wanted to learn.
Looping zone 4 out at the PIR doesn't cure the fault.
Looping zone 4 out at the control panel does cure the fault.
This suggests a wiring fault.
I believe the wires are hidden behind the coving at wall/ceiling height. I cannot see anything other than a thick cable coming out of the control box and disappearing into the coving, which then turns through 3-off 90 degree bends before going 30 feet down the hall towards the dining room.
Is it likely that this wiring will be clipped in place behind the coving? I don't know which would have been fixed first, probably when the house was being built.
I suppose i could just ignore it and carry on, since the alarm works as intended except for in the dining room.
I have once heard "rustling" as if from a mouse, but can't think how one would get inside the coving. I'll have a look upstairs but the only area with a few small places of access would be in the hot water cylinder cupboard upstairs.
How do installers usually fit alarm wiring in new houses? I'd hope that I could tie a string to the cable behind the PIR in the dining room and pull the damaged cable out towards the control box, using the string to then thread a new cable in.
More likely, I'll just ignore it. There is power to the PIR and also the tamper circuit works, so it's something with the 2 wires for NC and common.
Any help gratefully received, please.