Losing compression, may need a new engine

Vehicle is the XLT model. Fell out of cruise control twice and tire pressure light came on for a bit then off. Began running rough and check engine light came on. Mechanic changed oil and plugs. Light off then right back on. Then car when cold and starting up a hill became loud and rpm's were wacky. Mechanic spent hours trying to diagnose and cane to think the computer was the problem therefore had to take to dealership. They said initially that the valve cover gasket was cracked and oil had gotten into plugs so they replaced them (after one week after my guy replaced them). Diagnostics and plugs/coil $700.00. Now saying one cylinder losing compression and need new engine. My mechanic is not convinced and I don’t want to have to pay another dealership for diagnostics. They want almost $4,000.00. To replace engine which makes no sense given the year. Anyone have opinion about weather the dealership is correct? Any advice greatly appreciated. Older woman without a lot of money. Thanks