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

#define int long long

void solve() {
    int n, d, a, b;
    cin >> n >> d >> a >> b;
    int ans = min(d, n) * a + max(0LL, n - d) * b;
    cout << ans << '\n';
}

signed main() {
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    int T = 1;
    cin >> T;
    while (T--) {
        solve();
    }
    return 0;
}