#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;

int main(void) {
    ios_base::sync_with_stdio(false);

    int tests;
    cin >> tests;

    for (int i = 0; i < tests; i++) {
        int myH, liftH, mySpeed, liftSpeed;
        cin >> myH >> liftH >> mySpeed >> liftSpeed;

        cout << min(myH * mySpeed, abs(myH - liftH) * liftSpeed + myH * liftSpeed) << endl;
    }
    return 0;
}