GCF is the greatest common factor. its the value of the factors that are common to both the numbers being compared. In other words the product of the factors that are common to both numbers or the greatest factor common to both numbers.
factors of x⁴ = x * x * x * x = (x * x * x) * x
factors of x³ = x * x * x = (x * x * x)
factors common to both are (x * x * x) Therefore GCF is x³