#include<bits/stdc++.h>
using namespace std;

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