#include <iostream>
#include <cstring>
#include <algorithm>
#include <cstdio>
using namespace std;
int main()
{
      int T;
      cin >> T;
      int n, m, p, q;
      int ans;
      while(T--) {
            ans = 0;
            cin >> n >> m >> p >> q;
            int zu = n / m;
            int yu = n % m;
            if(zu == 0) {
                  ans = min(yu*p, q);
            } else {
                  if(yu == 0) {
                        ans = min(zu * q, n * p);
                  } else {
                        ans = min(zu * q + yu * p, min((zu + 1)* q , n * p));
                  }
            }
            cout << ans << endl;
      }
      return 0;
}