//
// Created by cteam24 on 11/27/21.
//
#include <iostream>
#include <algorithm>

using namespace std;

int main(void) {

    long long T, pos_me, pos_lift, speed_me, speed_lift, ans;
    cin >> T;
    for (int i = 0; i < T; ++i) {
        cin >> pos_me >> pos_lift >> speed_me >> speed_lift;

        if (pos_lift > pos_me) // lift is below me
            ans = min(pos_me*speed_me, pos_lift*speed_lift);
        else if (2*speed_lift < speed_me) // lift is too fast to use stairs
            ans = ((pos_me-pos_lift) + pos_me) * speed_lift;
        else
            ans = (pos_me-pos_lift) * speed_me + pos_lift * speed_lift;

        cout << ans << endl;
    }
    return 0;
}