Sounds like alot yu got there, hmmm i have seen engines that have evolved hair line cracks in the intake wall , especially on the #4. the thermostat and water through is right there. Also the gasket where the intake meets the cylinder head, have yu checked around there? There are coolant channels near the 4 cyliner and if the intake gasket is bad near the water passage that resides near that area a little water /coolant will seep into cylinder 4 keeping it frum idling and or running properlly. have yu checked yur coolant latelly? If the car never runs hot and is down a bit or so of coolant then maybe its going sumwhere huh? I would investigate the intake manifold gasket area and around the thermostat housing as it conjoines in that area.