Engine misfire

Hi there,

I have a cylinder four misfire issue that has been going on for some time. Started out changing plugs, swapping coils with no fix. Did a compression test and found that Cylinder four has about 110 lbs (compared to 150 on the rest). The engine starts and runs okay only misfiring below 2500 rpm and is audible on start up and idle speeds. I know that this could be a ring issue or valve issue. My question is that if I have driven this for 30,000 miles plus in this condition, do you think its likely that the cylinder has some damage? I have another engine lined up (used, 40,000 miles) for $2,400.00 which is not too bad. I do not want to tear into this to fix it and realize that it is much bigger job than I thought. Plus, if I bought the engine I would have spare parts. Thoughts?

Thank you,

Steve