How does one get a four in an exponent here? Is there a link to the math? Edit : cool link below, but includes the disclaimer “The accuracy of the law of the

How does one get a four in an exponent here? Is there a link to the math? Edit : cool link below, but includes the disclaimer “The accuracy of the law of the fourth power is disputed among experts, since the test results depend on many other factors, such as climatic conditions, in addition to the factors mentioned above.”