I went through this a few years ago.
I think I wrote it up at the time... maybe, maybe not.
Anyway, by setting about trying to quickly test them individually for current draw I quickly realized that they are powered from a thick copper bus bar that their heads are screwed directly into.
It MIGHT have been possible to measure the current going to all 4, then the last 3, the last 2, last one, etc and by subtraction figure which were flakey.
I didn't do that, instead I unscrewed them from the bar and removed them, took them to a service shop - along with the injectors to get them checked out.
They all checked out fine, so I cleaned everything up and re-assembled it.
I think that might have solved it, just cleaning the contacts between the glow plugs and the copper bus bar.
Careful clean re-assembly has fixed many problems for me over the years, so that is my working assumption on things like this - include a dab of silicon (errr,,, "dielectric grease" ?) on each, etc.
The injectors were a mixed story, I think a couple of places quoted me more to re-build them than their full new dealer price and a couple of them said they were fine.
Anyway, I similarly put those back with no further work and the tractor has started and run fine ever since.