#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 (or at the same level - no difference but accounted for)
            ans = min(pos_me*speed_me, pos_lift*speed_lift);
        // lift is above me from now on
        else if (2*speed_lift <= speed_me) // lift is too fast to use stairs (or equally fast - no difference but accounted for)
            ans = ((pos_me-pos_lift) + pos_me) * speed_lift;
        else // lift is faster than me, but not enough to do two trips
            ans = (pos_me-pos_lift) * speed_lift + pos_lift * speed_me;

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