Step 1: Write the general terms.
Let the G.P. be $a, ar, ar^2, ar^3, ar^4$.
Step 2: Use the given condition.
The third term is $ar^2 = 4$.
Step 3: Find the product of first five terms.
\[
a \cdot ar \cdot ar^2 \cdot ar^3 \cdot ar^4 = a^5 r^{10}
\]
Step 4: Express in terms of the third term.
\[
(ar^2)^4 = 4^4
\]
Step 5: Conclusion.
The product of the first 5 terms is $4^4$.