#include <iostream>
using namespace std;

int main() {
    int T;
    cin >> T; 
    
    while (T--) {
        long long N, D, A, B;
        cin >> N >> D; 
        cin >> A >> B; 
        
        long long totalCost = 0;
        
        if (N <= D) {
       
            totalCost = N * A;
        } else {
           
            totalCost = (D * A) + ((N - D) * B);
        }
        
        cout << totalCost << endl; // Output the total cost
    }
    
    return 0;
}