#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++) {
        ull myH, liftH, mySpeed, liftSpeed;
        cin >> myH >> liftH >> mySpeed >> liftSpeed;

        if(myH < liftH) {
            cout << min(myH * mySpeed, liftH * liftSpeed) << endl;
        } else {
            // first of all, call lift here
            cout << min(myH * mySpeed, (myH - liftH) * liftSpeed + myH * liftSpeed ) << endl;
        }
    }
    return 0;
}