#include<iostream>
using namespace std;

int main() {
  int t;
  cin >> t;
  for (int i = 0; i < t; i++ ) {
    int l, r, a, b;
    cin >> l >> r >> a >> b;
    if (a < 0) {
      cout << a * l + b << endl;
    }
    else {
      cout << a * r + b << endl;
    }
  }
}