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

void solve() {
    int n, d; 
    cin >> n >> d;
    int a, b; 
    cin >> a >> b;

    // Check if nights spent is less than or equal to the first D nights
    if (n <= d) {
        cout << n * a << endl;  // All nights are charged at the rate 'a'
    } else {
        cout << (d * a) + (n - d) * b << endl;  // First D nights at 'a', rest at 'b'
    }
}

signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);

    int test;
    cin >> test;
    for (int tc = 0; tc < test; ++tc) {
        solve();
    }

    return 0;
}