Let f: r rbe a differentiable function with f(0) = 1 and satisfying the equation fx+ y) = f(x)f (y) +f(x)f(y) for all x, y e r. then, value of log, (f(4)) is

Let f: R Rbe a differentiable function with f(0) = 1 and satisfying the equation fx+ y) = f(x)f (y) +f(x)f(y) for all x, y e R. Then, value of log, (f(4)) is