Almost guarantee you the #4 glow plug you took out that you thought was wet with oil is actually soaked in fuel. Odds are your injectors have never been rebuilt, and after a few hundred thousand miles and over two decades, they are beginning to leak... not to mention which, you have been cranking it over and trying to start it. The front three glow plugs were dry when you took them out because they are still live, according to your resistance measurements, so they have been getting hot enough to burn the fuel off of them when it gets sprayed down by the injector above. I'll bet money that the #4 plug is dead, and that is why it is wet, and also part of why the motor is not starting.
I think your problems are probably not as serious as you think they are. Despite their reputation, these engines are actually pretty tough units as long as they are not abused (which really means only three things: use of low-quality non-synthetic oil, overheating, or neglect of routine maintenance such as timing belts, valve adjustments, glow plugs and injector rebuilds). In the wintertime, when the cold weather sets in, there is always a rash of people thinking their car has finally died because it won't start anymore. Notice that here in the Pacific Northwest (I'm a few hours north of you in Seattle), we are experiencing temperatures that are about as cold as it ever gets around our region. These are the kinds of conditions that test your diesel's cold starting ability, which means timing, valves, injectors, and most of all, glow plugs. If an IDI diesel has good glow plugs in all cylinders it WILL start even if it is very worn, assuming other factors (timing, valves, fuel etc) are in spec. I am virtually certain engine wear is not the cause of your sudden failure. It might be an ongoing issue in your motor's life, but I believe it is not what caused this particular episode.
If it was smoking a particularly large amount leading up to the no-start, I would be looking more and more at fuel quality and at engine timing. Is the crank bolt loose, allowing the timing belt drive sprocket to slip? What about the front or rear cam sprocket bolts? If any of those works loose enough for the sprocket to move, the engine will come out of time and not start. I mentioned before about doing a rudimentary visual check of basic timing, which requires no tools and will ensure that the timing is at least close enough for the engine to start. Give that a try and see what you find out.
Smoke is also caused by incorrectly adjusted valves, and if they are severely out of adjustment it can cause a loss of compression. As the valves and valve seats wear, the valve lash DECREASES (contrary to what most people assume). The valves get tighter, not looser. If neglected, they eventually tighten up to the point that they are not completely closing, and that causes major issues since the cylinder seal is lost and achieving full compression is impossible. That might be why you experienced additional smoke and felt so little compression from one of your cylinders. If you don't know whether the valves are properly adjusted, get a set of angled feeler gauges for a few bucks and check it. Takes only about 10 minutes.
If one or multiple cylinders are low on compression, you will be able to hear it as the engine cranks, since the cranking speed will be irregular. Do you hear an abnormal rhythm as it cranks on the starter?
What kind of fuel do you have in it? If it's diesel, is it winterized to deal with cold temps? If it's biodiesel, could the cold weather have gelled it up?
Don't give up hope and switch to the dark side. A gasser swap is not the answer! I think you're looking at something simpler than you expect, like a failed injector, a bunch of bad glow plugs, wrong timing, wrong valve lash, or some bad fuel. D24Ts do wear out, but not in a week. That is still considered a very short progression of symptoms... yes, I almost would call it an "instant problem." Cover all the basics before you assume the worst. I believe your diesel will ride again.
