I haven't read the Four Agreements yet, but I surely will in the coming days. Based on what I understood from the point you were trying to make, I remembered a short story that I read some time back.
So there is this 30 year old man, who is diagnosed with cancer. The doctor tells him, he is not going to live for more than 12 months. And eventually, the man dies in 12 months.
The same doctor now, in a similar case, predicts that a 5 month old girl, is not going to survive for more than 12 months. The girl lives for 5 years. And 5 years later, the girl dies, but not off cancer.
Now the difference between the two cases is that the 5 month old girl was too young to understand life, death and truth. Whereas the grown man, had accepted the doctor's truth as his truth and had made himself believe that he was not going to survive for more than 12 months.
And I agree, this is a huge idea. It could actually make or break your life.