#include <bits/stdc++.h>
using namespace std;

#define FAST ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
using ll = long long;

int main() {
  FAST;
  
  int tc = 1, ti;
  cin >> tc;

  for (ti = 1; ti <= tc; ++ti) {
    ll n, d, a, b, ans;
    cin >> n >> d;
    cin >> a >> b;

    ans = 0;
    if (n < d) {
      ans += n*a;
    } else {
      ans += d*a;
      n -= d;
      ans += n*b;
    }

    cout << ans << "\n";
  }

  return 0;
}