#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;
    cout<<(d*a)+(n-d)*b<<endl;

}

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

    int test = 1;
    cin >> test;
    for (int tc = 0; tc < test; ++tc)
    {
        solve();
    }

    return 0;
}