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

namespace solve {
  namespace test_case {

    void main() {
      int n, d, a, b, ans; cin >> n >> d >> a >> b;
    
    if (n <= d) {
       ans= n * a;
    } else {
      ans= a * d + (n - d) * b;
    }
    cout << ans << '\n';
    }
  }

  void main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int t; 
    cin >> t; 
    while (t--) {
      test_case::main();
    }
  }
}

signed main() { solve::main(); }