#include <stack>
#include <cmath>

using namespace std;

int main()
{
double n;
stack<double> numbers;

while (scanf("%lf", &n) != EOF)    numbers.push(n);
while (!numbers.empty())
{
    printf ( "%.4f\n", sqrt( double(numbers.top()) ) );
    numbers.pop();
}

return 0;
}

Yep!
Read and parse input manually, scanf is expensive, %lf is even more expensive. As a bonus, you would not have to store read doubles. What surprised me is that printf is even much more expensive, so if you can create text representation of a double in given format - you are welcome!
I have just got 0.031s execution time, what is yours?