#include <iostream>

using namespace std;

int main (){

    int T;
    cin >> T;

    for (int i = 0; i < T; ++i) {

        int N, D, A, B;
        cin >> N >> D;
        cin >> A >> B;

        if (D >= N) {
            cout << A*N << endl;
        }
        else {
            cout << (A*D)+((N-D)*B) << endl;
        }
    }

    return 0;

}