#include<bits/stdc++.h>
using namespace std;
#define optimize() ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define endl '\n'
#define ll long long int
int main()
{
    optimize();
    ll t;
    cin >> t;
    while(t--){
    ll a,b,c,d;
    cin >> a>>b>>c>>d;
   cout <<(b*c)+((a-b)*d)<< endl;
    }
 }