Compression test gauges should have a check valve that keeps the pressure in them so you can read the gauge after cranking. The valve in the tester you have isn't working probably.
The readings on the first three cylinders are good. So the gauge is fine for accuracy I think. If the wet and dry on four show 80 then you have a burned valve more than likely. It's not the guides except that worn guides allow the valve face and seat to not match up. Exhaust guides can wear really badly on these engines.
Pull the cam cover to check for a broken spring. You'd have about 1/8 inch lash or more on the bucket. And lots of noise.
Depends on how much oil you put in the cylinder as well. If not enough then a busted ring won't get sealed. Still could be a busted ring plowing a groove in the cylinder.
So you either drive it to a bigger failure, or pull the head to see. If it's a valve fix it. If it's rings, find another engine. Pistons are expensive.