#include <iostream>
#include <vector>
#include <cmath>

using namespace std;

typedef long long int ll;

int main() {
    ios::sync_with_stdio(false);

    long long int factorials[10] = {1,1,2,6,24,120,720,5040,40320,362880};

    long long int cases;
    cin >> cases;

    for (int k = 0; k < cases; ++k) {
        ll y, l, ty, tl;
        cin >> y >> l >> ty >> tl;

        ll ans = 0;
        if (y <= l) {
            ans = min(ty*y, tl*l);
        } else {
            ll delta = (ty*(y-l))/(2*tl + ty);
            ans = min(ty*y, ty*(y-l)+tl*l);
            ans = min(ans, (y-l-delta)*ty+(l+delta)*tl);
        }
        cout << ans << endl;
    }

}