#include <iostream>
#include<vector>
#include<iomanip>
#include <math.h>
using namespace std;
int main()
{
    double a;
    vector<double> vd;

    while(cin>>a)
    {

        vd.push_back(a);
    }

    cout.setf(cout.showpoint);
    setprecision(4);
    setiosflags(ios::fixed);

    for(vector<double>::size_type i=vd.size()-1;i!=-1;--i)
    {

        cout<<pow(vd[i],0.5)<<endl;
    }

    return 0;
}

setprecision and setiosflags didn't have any effect in your code. You need to put them in some 'cout << ' line.

Try to change these lines
cout.setf(cout.showpoint);
setprecision(4);
setiosflags(ios::fixed);
to
cout.setf(ios::fixed);
cout.precision(4);