Markov Inequality Upper Bound

Suppose X is a random variable such that $$E[2^X] = 4$$. Give an upper bound for P(X ≥ 3).

I know I must use Markov's inequality here: P(X ≥ a) = $$\frac{E|X|}{a}$$

For other problems I have solved I was given the expected value not as a function of X so I am unsure how to manipulate this in order to give the desired bound.

$$X \geq a$$ iff $$2^{X} \geq 2^{a}$$. Hence $$P(X \geq a)=P(2^{X} \geq 2^{a}) \leq \frac {E2^{X}} {2^{a}}=\frac 4 {2^{a}}$$. Put $$a=3$$.