1. **State the problem:** Find the value of $a^2$ when $a=4$. 2. **Formula used:** The square of a number $a$ is given by $a^2 = a \times a$. 3. **Substitute the value:** Replace $a$ with 4. $$a^2 = 4^2$$ 4. **Calculate:** $$4^2 = 4 \times 4 = 16$$ 5. **Answer:** The value of $a^2$ when $a=4$ is $16$.