#include <iostream>
using namespace std;

int main() {
    int T;
    cin >> T;

    while (T--) {
        int N, D;
        cin >> N >> D;
        int A, B;
        cin >> A >> B;

        long long total_cost;
        if (N <= D) {
            total_cost = 1LL * N * A;
        } else {
            total_cost = 1LL * D * A + 1LL * (N - D) * B;
        }

        cout << total_cost << "\n";
    }

    return 0;
}