Thanks for the insights. I did use a mechanic's stethoscope and, while the knock reverberated through the engine (I could hear it everywhere) it was definitely most pronounced right around the combustion chamber for #4. Don't know if that's valve or piston (it's not injector). The points Hercules makes about carbon and valve tightness are good ones -- #4 intake was tight when I originally checked it. I'll remove the valve cover and look closer at what's going on there, recheck clearance. Any way to check for burnt/bent valve without removing the head?