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

int main(){
  int t;
  cin >> t;
  while (t--){
    ll n, d, a, b;
    cin >> n >> d >> a >> b;
    ll mn = min(d, n);
    ll ans = mn * a + (n - mn) * b;
    cout << ans << endl;
  }
}