#include <iostream>
#include <algorithm>
#include <vector>
#include <math.h>

using namespace std;

int main()
{
    int num_cases;
    cin >> num_cases;
    // uint64_t result = 0;

    for (int i = 0; i < num_cases; i++)
    {
        int my_floor, lift_floor, my_floortime, lift_floortime;
        cin >> my_floor >> lift_floor >> my_floortime >> lift_floortime;

        // below the lift
        if (my_floor > lift_floor)
        {
            // if I am faster than lift
            if (my_floortime < lift_floortime)
            {
                cout << my_floor * my_floortime << "\n";
            }
            else
            {
                int combined_travel = (my_floor - lift_floor) * my_floortime + lift_floortime * lift_floor;
                int lift_travel = ( lift_floor + 2*(my_floor - lift_floor) ) * lift_floortime;
                cout << min(combined_travel, lift_travel) << "\n";
            }


        }
        // above the lift => only call the lift
        else
        {
            if (my_floor * my_floortime < lift_floor * lift_floortime)
            {
                cout << my_floor * my_floortime << "\n";
            }
            else
            {
                cout << lift_floor * lift_floortime << "\n";    
            }
        }



        
        
    }
    


    return 0;
}