1. The problem asks to identify which of the given functions is an exponential function. 2. An exponential function has the form $$y = a^x$$ where the base $$a$$ is a positive constant not equal to 1, and the exponent is the variable $$x$$. 3. The first function is $$y = 4^x$$. Here, the base is 4 (a positive constant) and the exponent is $$x$$, so this fits the definition of an exponential function. 4. The second function is $$y = x^4$$. Here, the base is the variable $$x$$ and the exponent is a constant 4, which is a power function, not an exponential function. 5. Therefore, the exponential function among the two is $$y = 4^x$$. Final answer: $$y = 4^x$$ is an exponential function.