#include <bits/stdc++.h>
#define int long long
#define endll '\n';
#define pb push_back
#define all(v) v.begin(), v.end()
using namespace std;


const int mod = 1e9 + 7, N = 1e6;
 
void solve()
{
 
 int n , d; cin >> n >> d;
 int a , b; cin >> a >> b;
 int ans = 0;
 if(n <= d) {
   ans = n * a;
 }
 else {
   ans = d * a;
   n -= d;

   ans += b * n;
 }

 cout << ans << endll;
}

int32_t main()
{
  ios::sync_with_stdio(false);
  cin.tie(0);
  int t = 1;
  cin >> t;
  while (t--) {
   solve();
}
 return 0;
}