How do I go about tackling the problem below?

$\displaystyle

{\rm If }y = 4^x {\rm then }\frac{{{\rm dy}}}{{{\rm dx}}} = k.4^x {\rm where k is: ?}

$

Is started with trying first principles and came up with this

$\displaystyle

\mathop {{\rm lim}}\limits_{{\rm h} \to {\rm 0}} \frac{{4^{a + h} - 4^a }}{h}

$

I'm stuck