#include <iostream>
using namespace std;

int main()
{
    int t;
    cin >> t;
    while(t--){
        int a, b, n, d;
        cin >> n >> d;
        cin >> a >> b;
        int total_cost = 0;
        if(n <= d){
            total_cost = n*a;
        }
        else{
            total_cost = (d*a)+(n-d)*b;
        }
        cout << total_cost << endl;
    }
}