#include<bits/stdc++.h>
using namespace std;
int main()
{
   int tc;
   cin >> tc;
   while(tc--){
    long long n, d, s, M, m,sum;
    cin>>n>>d;
    s=n-d;
    long long a, b;
    cin>>a>>b;
    M=d*a;
    m=s*b;
    sum=M+m;
    cout<<sum<<endl;

   }
    return 0;
}