/*
 *   Copyright (c) 2024 Emon Thakur
 *   All rights reserved.
 */
#include<bits/stdc++.h>
using namespace std;
using ll = long long;
int main()
{
    int t; cin >> t; while(t--)
    {
        ll n,d; cin >> n >> d;
        ll a,b; cin >> a >> b;
        ll ans = a*min(d,n) + max(n-d,0ll)*b;
        cout << ans << endl;
    }
}