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Yes, approximately a 25% drop in barometric pressure.
Y'd see the same percentage drop on a Pressure gauge
not just a 4 PSI drop -- I wish, as it would be a lot easier
motoring up in the Sierra's

Some even push compression ratios up to 14:1 if they
operate @ high elevation -- and "ping" free with regular.

As you repair your engine, I would also give a good look to the carburetors, especially the four by-pass holes. This holes are different in diameter and I found them also clogged when I bought my 710M - when these holes are clogged for instance on one carb the engine is running poor on idle and sometimes missing out on exceleration from idle speed upwards.
Your piston nr.four had a hole in and now the exhaust valve is burnt - so it might be a good idea to check those four bypass holes. Also the jets are easy to access for inspection.
Those four bypass-holes may be partly clogged with carbon, so blowing it free with compressed air may not help, but to use a drill you may first need to know the original diameter, so that the holes do not get inlarged.

[attachment=0]Pinz Vergaser 2 Unterteil 00242x.jpg[ /attachment]

@Pinzengauer: "does that insulation have to be on there?"

If you mean those insulations with part nr. 712.1.08.007.1 or 712.1.08.525.1, yes, they are necessary to protect the carbs against heat from the engine, as air cooled engines get much hotter than water cooled engines.

@heinz

Thanks, Ill check those bypass holes in the carbs. I doubt they're plugged as I got them refurbished and only have a few thousand miles on them, but I will definitely take a look.

Also, that explains the purpose of the insulation on the intake manifolds, I appreciate the information!

Well if the carbs are refurbished, these holes should be clean and open in the right diameter - you don´t have to dismentle the carbs for checking those holes - just take out the four selenoids and if the carbs are monted on the engine, take a mirror and good light and check through the selenoid holes. The bypass holes are well visable through the selenoid holes.
Sometime I found one of the holes blocked by some material like silicone, used when the components have been assembled - such things can be removed by compressed air.

When I´ve got my Pinz, at the beginning, I had to clean these holes and also idle and main jets from time to time because the engine started to run poor in idle and slightly above.

When I take out the sparks for some reason I always check the spark plug face to detect any blockage in the carbs and on which cylinder I have to check (for this reason I have marked my sparks from 1 - 4).