Last year two of my brothers died two days apart with different stage 4 cancers.
I sure butchered that sentence but I hope you understood it.
It means it has spread from the original site. While often that may mean it is not totally curable, it is often treatable. People can and often do live longer with targeted treatments.
That's almost always the case but there are rare exceptions. Jimmy Carter was one such. His stage 4 melanoma had spread everywhere .. until he was given an experimental drug known as a 'checkpoint blocker'. The idea is.. the body's immune system is ready to destroy the cancer wherever it is but a 'lock' is holding it back. The drug releases that lock. And sure enough his cancer vanished and he lived to be 100.
“I understood that stage 4 cancer means it had metastasized such that there no way to expect any therapy to would be any help.”
It had spread to his bones. The referenced treatment would do nothing except maybe shrink a couple of tumors in his liver.