#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
// #define cerr cout
#include "algo/debug.h"
#else
#define deb(...) 30
#endif
#define ll long long
#define all(x) (x).begin(), (x).end()
#define f(i, n) for (int i = 0; i < n; i++)

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    int t;
    cin >> t;
    while (t--)
    {
        ll n, d, a, b;
        cin >> n >> d >> a >> b;
        if (n <= d)
            cout << n * a << endl;
        else
            cout << (d * a + (n - d) * b) << endl;
    }

    return 0;
}