+1 on the headgasket. If it only leaks when overheated it's probably just coming out the radiator overflow.
There are a few diagnostic things you can do. A compression test is top of the list. What you're looking for is a large difference between the cylinders.
Say where the compression is 135, 130, 140, 70. That'd indicate major problems in number 4.
The exhaust gas test for coolant is another. You can find those kits all over. With that, you fill a test tube with a reagent and then you put a cork with a tube leading off it into the radiator cap outlet, to pipe gas from the cooling system into the test tube so it bubbles in the reagent. If the reagent turns yellow, combustion gasses are present.
Head Gasket or Combustion Leak Test Procedure (Gasoline Engines Only)
I think most auto parts stores would have a kit like this, as does Harbor Freight.
Also take a good look at both your oil and coolant to see if there's any contamination (where either looks like a "milkshake").
If all those tests are negative, then maybe look at your thermostat. You can diagnose that easy enough with an infrared thermometer (to see if it's opening).
But with the symptoms you have - all that excess air pressure - I'd put my money on the headgasket.