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

void solve()
{
    int n,d,a,b;
    cin>>n>>d;
    cin>>a>>b;
    int total=(d*a)+((n-d)*b);
    cout<<total<<endl;
}

int main()
{
    int t; cin>>t;
    while(t--)
    solve();
}