zero compression in #4 cylnder 2008 Silverado

If it's just the one cylinder showing no compression, take off the corresponding valve cover and look for broken valve springs on either the intake or exhaust valves. Also look for a push rod or tappet jumped off at high RPMs. Those things are not too serious.

Before jumping to the conclusion it's a blown head gasket, try removing the valve cover and checking if it's a dropped valve.. Slack in the valve train/rocker can cause that. Some engines rockers can even pop off (like dodges). Check the lifter as well. If valves and lifter look okay, then try a bore scope down the cylinder, and check if piston has a hole in it (it does happen). A leakdown test can also help check condition of piston rings. If that doesn't reveal anything, then you are likely looking at a blown head gasket.

I swear Hornet your post wasn't showing before I wrote mine, lol. (I'm a slow typer).

I understand. And you had a lot more typing to do. I've had that happen to me many times and it makes the author look bad. But in your case, your wealth of knowledge is very impressive and I learn a lot from you. You know what they say....brilliant minds think alike!