#include <iostream>

using namespace std;

int main() {
    int T;
    cin >> T;
    
    for (int t = 0; t < T; ++t) {
        int N, D;
        cin >> N >> D;
        
        long long A, B;
        cin >> A >> B;
        
        // Calculate the total accommodation fee
        long long total_fee = 0;
        
        // Cost for the first D nights
        total_fee += D * A;
        
        // Cost for the remaining (N - D) nights
        if (N > D) {
            total_fee += (N - D) * B;
        }
        
        // Output the total fee for this test case
        cout << total_fee << endl;
    }
    
    return 0;
}