#include <bits/stdc++.h>
#define ll long long
using namespace std;
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int t;
    cin >> t;
    while (t--)
    {
        ll a, b, c, d;
        cin >> a >> b >> c >> d;
        cout << (b * c) + ((a - b) * d) << endl;
    }
}