#include <iostream>

using namespace std;

int main() {
    int t, n, d, a, b;
    cin >> t; // Read the number of test cases
    for (int i = 0; i < t; i++) {
        cin >> n >> d; // Read the number of nights and the number of first nights with higher rate
        cin >> a >> b; // Read the cost per night for first D nights and subsequent nights
        // Calculate the cost for the first D nights or for all N nights if N <= D
        int firstPart = min(n, d) * a;
        // Calculate the cost for the remaining nights if N > D
        int secondPart = max(n - d, 0) * b;
        // Total cost is the sum of the above two parts
        int totalCost = firstPart + secondPart;
        cout << totalCost << endl; // Output the total cost for this test case
    }
    return 0;
}