There's a theory floating around that the dropped valve seat didn't start showing up til EGR was equipped. And EGR reintroduces exhausts gases at the #4 intake runner. Higher intake temps on #4.
Wives tale..... Idk.
Member of the Dropped Valve Seat Club.
Replaced #4 Piston, Rings, Rod+Bearings, AERA Re-Manufactured Cylinder Head and a light Hone. Then CLEANED the hell out of everything.