Funny that,
Mine also went between 3 and 4 at the Daddy thrash!

I was pulling 7000rpm in top when it let go. it also blew into the oil way so dumped loads oil around the engine bay. Running 8.8:1 CR and 25psi boost.

Before I had to fit the Felpro I was running with a copper head gasket and sealing rings from Ferriday, that coped with 2bar+, however it had been reused 5 times so thought I better retire it.

Simon.