#include <iostream>
#include <vector>
#include <cmath>
using namespace std;

int main()
{
    long long val = 0;
    vector<double> v;
    while (cin >> val &&val!=EOF)
    {
        v.push_back(sqrt(val));
    }
    for (int i = v.size()-1; i >=0; i--)
    {
        cout<< v[i] << endl;
    }
    return 0;
}

Set your val type to double

for (int i = v.size()-1; i >=0; i--)
    {
        cout << fixed << setprecision(4) << v[i] << endl;
    }

use this. your program is supposed to print 4 points

try to change "long long val" to "double val"

for (int i = v.size()-1; i >=0; i--)
    {
        cout << ios::fixed << setprecision(4) << v[i] << endl;
    }