OK..my mechanic has taken the head off...machined it and still have a loss of compression in the #4 cylinder...has this happened to any of you?

Sorry...forgot to mention it's a 1999 Dodge Ram 1500.

Before the #4 cylinder went I heard a lot of pinging sounds coming from the engine.

Was any other work done to it? If he machined the head, great. You could have a bad set of rings in that cylinder. How many miles on the truck?

You need to to a leak down test on the #4 cly..Did your mechanic check the piston good before putting head back on? could have burnt a small hole in the piston ..the ping noise is actually chewing away at a piston every time it pings.
Dave