Here is a theory that I can provide some backup for:
The #4 intake cam is the first one to pit because of insufficient lubrication. I have also heard (and saw some evidence) that ZX11s do the same thing. My bike pitted three sets of cams in 40k miles all the while using Amsoil and Mobil 1 oils.
The backup? Look at the pictures:
My ZRX rocker tubes:
Tubes from spare ZX11 head:
I had another set from another ZX11 head that looked the same.
The oil holes on #1-3 are larger in diameter and aren't blocked as much.
The intake tubes are at the bottom of both pictures. Notice the wear marks from the rocker arms and the position of the oil holes. Since the #4 cylinder area has holes on only one side of the tube and the rest of them have holes on both sides, it is clear that little to no oil is getting into the rocker oil hole to lube the cam lobe. All of the holes are offset the same amount so I believe that the holes bored into the cam towers for the tube were bored too far into the intake side. I will be adding a 1mm shim to the intake side to help line up the holes with the rocker oiling area.
It would be real interesting to see the condition of other engines that have eaten intakes cams.