Sounds a lot like a leaking head gasket or a cracked cylinder head. I just went through what sounds like the same problem with mine. After running it like that for a few months it started burning coolant pretty quickly so I had to tear it apart right after my manual swap.
Have you checked for moisture under the oil cap? (milky looking oil)
Have you noticed any white smoke or does the exhaust smell like coolant/ have more water coming out than normal condensation?
That coolant in your reservoir is going somewhere is it a tank leak or is it just "vanishing"?
To diagnose it better you can check those things and pull the codes if there are any and reference this post for an idea
OBD II CODES- for those without a scanner
But if it is having cold start compression issues until it warms up and runs good once it's warm its likely a coolant leak into that #4 cylinder you said had the gnarly plug. The engine is struggling to burn the fuel in that cylinder with the water until it blows it out the exhaust then when its running better the engine heats up and seals up your leak when everything expands.
To know for sure you'd have to pull the heads or have a good shop do it for you if you got the $$$. Theres a lot of good stuff online on how to DIY though if you're inclined to wrench on your truck yourself.