#include<bits/stdc++.h>
using namespace std;
#define fast ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define dbg(a,b,c,d) cerr<<a<<"  "<<b<<"  "<<c<<"  "<<d<<endl;
#define kill(a) {cout<<a<<endl;continue;}
#define KILL(a) {cout<<a<<endl;return 0;}
#define debug cerr<<"Error Found"<<endl;
#define mem(a,b) memset(a,b,sizeof(a))
#define lcm(a, b) (a/__gcd(a,b))*b
#define w(t) cin>>t;while(t--)
#define pi  2 * acos(0.0)
#define endl "\n"
int t, cs = 0;
const int mxn = 2e5 + 3, mod = 1e9 + 7;

int32_t main()
{
   //fast
   w(t)
   {
       int n, d;
       cin >> n >> d;
       int a, b;
       cin >> a >> b;
       int64_t cost = 1LL * min(d, n) * a + 1LL * max(0, n - d) * b;
       cout << cost << endl;
   }
}