/**
 *    author:  pollob18
 *    created: July 2024
**/
#include <bits/stdc++.h>
#define ll   long long int
#define yes  "YES"
#define no   "NO"
#define nl   "\n"
#define pb   push_back
#define all(a)  a.begin(),a.end()
#define allr(a) a.rbegin(),a.rend()
#define Code_By_pollob ios::sync_with_stdio(false); cin.tie(0);
using namespace std;

void solution()
{
    ll n,d;
    cin>>n>>d;
    ll a,b;
    cin>>a>>b;
    if(n<=d){
        cout<<n*a<<nl;
       return;
    }
    ll x=d*a;
    ll y=(n-d)*b;
    cout<<x+y<<nl;
}
int main()
{
    Code_By_pollob
    int tc;
    cin>>tc;
    while(tc--)
    {
        solution();
    }
    return 0;
}