#include <iostream>
using namespace std;

// Function to calculate the total accommodation fee for each test case
int calculate_fee(int N, int D, int A, int B) {
    if (N <= D) {
        return N * A;
    } else {
        return D * A + (N - D) * B;
    }
}

int main() {
    int T;
    
    // Input the number of test cases
    cin >> T;
    
    // Process each test case
    for (int i = 0; i < T; i++) {
        int N, D, A, B;
        
        // Input N (total nights) and D (discount nights)
        cin >> N >> D;
        
        // Input A (higher rate) and B (lower rate)
        cin >> A >> B;
        
        // Output the total accommodation fee for this test case
        cout << calculate_fee(N, D, A, B) << endl;
    }

    return 0;
}