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Look at the picture. There is almost no ridge between number 4 cylinder space and the surrounding head surface.

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That is interesting. I can't speak to what happened, or why, but I think in general if there are going to be troubles with any cylinders, #4 & #5 are top contenders.

It is very interesting but it must be sealing OK if the engine was running. Assuming no cracks etc, the usual issue will be if the head been milled too much and a valve head will hit. If the head is the one removed and has not been machined, if you had no previous issues chances are it is OK.

To check a head for too much milling after coming back from the shop, you can install the lifters and valves without springs but if already properly installed, leave them as is which would give a more accurate estimation of maximum lift. Place a thin layer of modeling clay in the pockets above the valves and carefully lay the head over the studs so it lays on the block but without a head gasket or any plugs in the holes. Do not use any nuts on the studs.

Turn the engine over slowly by hand so all the valves will go thru a complete cycle and watch for any movement whatsoever from the head. If movement is seen, chances are good a valve is hitting. If no movement seen carefully lift the head off and examine the clay for crescent shaped depressions from the edge of the valve. If clay is marked but still intact, with a headgasket you will probably be OK but if the clay is mashed thru and so thin the head metal is visible it becomes iffy particularly if the car has hydraulic lifters that will get pumped up.

A tiny bit of additional clearance can be had above a valve by grinding a very slight groove in the head where the crescent shape is apparent. Only a tiny bit of grinding though or you can cause more trouble and wind up scrapping the head. IMO, it is a good idea to do this test on any unknown head, particularly a used one which you bought off ebay and hope is better than one you had.. Several have had new valve jobs, freshly obtained or perhaps machined heads installed and torqued only to try starting the engine and find a valve has hit the head and something is now damaged.

I have stared at the photo and am too obtuse to see what you are talking about. What ridge? The head should be a flat milled surface and that is what I am seeing in the shiny spot.

Were there signs of leakage past the gasket?

Then I'm obtuse too as I also couldn't see anything out of the ordinary. If you know how, maybe circle or otherwise mark it on the image for clarity!

I must be acute. Kunzea is saying that this edge here is not as defined as the other cylinders, like it has eroded away.

Hi acute, I believe that you have highlighted cylinder 5 above the piston....look one over to the right and enhance....there appears to be severe erosion. I would not be reusing that cylinder head. I have never seen another one like that. Peter Toet

Yeah, something weird going on here.

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Oh yeah, I got distracted by the spot that had been cleaned off. I appears to have happened here some as well. Even if it was a bad gasket I can imagine this happening..............maybe bad casting?

Regardless, I would agree, I think Kunzea should look for a replacement head.

I'm still staring at these photos on my monitor and not getting it at all. If there is a low spot, why not just have the head surfaced? Otherwise, I'll be happy to sell the man a head.