If you take those 25 year old injectors out to be tested I would be very surprised if the results say 'perfect working order within manufacturers specification' so you now have a bill for testing and another for the repair of 4 injectors which could turn out to be nothing to do with the smoke when refitted!
You need to narrow the fault down as Jack states.
Could you borrow a hand held infrared temperature indicator, if pointed at the exhaust manifold next to each exhaust port the one not burning will be significantly lower temperature then the others without even putting a spanner on the engine!
Double check this then by cracking injector pipes, be carful though I am not knowledgable about the pressures used, high pressure diesel cutting your finger off is not good!!
Then swap injectors around in the engine, always mark things up though so you can put everything back as it was, last thing you want is to introduce a new fault during diagnosis.
If the fault has moved then yippee get an injector or repair, I'd be maybe then consider a refurb on them all if the same age and you need to know why, if its water in your diesel it might be needing all 4 again in a few weeks.
If the fault does not move with the injector then put it back where it started from and maybe a compression check next, could be a sticking valve.
Again try to pin it down, if it shows up as low compression on No 4 then take the rocker cover off and is it possible to measure the valve stem lengths to see if one does measure low compared to the same valves on the other 3 cylinders.
You need to be methodical and certain that once you have found something you can then prove it beyond reasonable doubt before spending your hard earned money.
I am trying to get my head round why it would go away once warm though, this must be a big clue, if low on
compression why would it do that, does the valve free up again once warm??
I had e vehicle once run on 3 every morning, I worked out it must have been leaking water into the bore of one cylinder when parked up, after a few miles the water was cleared and it ran ok, I removed the coolant expansion tank cap when I parked up one night as soon as I could safely do so and then refitted the next day, vehicle started up on 4 straight away as there had been no pressure to force water in all night long.
Replaced head gasket as i found a corroded part between the water jacket and No 2 which i had already proved buy removing each HT lead whilst it was running on 3, don't want to do job and find it was a cracked liner or head do you?
It's all about investigation and proving what you have found and a reason for it doing it, no point in repair lasting 2 weeks is there?
A garage will not do this, they will swap parts at your expense using little knowledge and a lot of '"well last time we had one do this it was the pump"
Fault still the same, they'll tell you the pump was definitely shot you have a shiny pump for £500 and still smokey!
Now what, swap 4 injectors, then the pipes, then they'll take it to an expert who will start doing what you are going to do after reading this!
Good luck, keep us posted each time you do anything and the result of it.
Eventually it will be apparent exactly what it is.