#include <iostream>

using namespace std;

void solve() {
  int a, b, x, y;
  cin >> a >> b >> x >> y;

  if (a > b) {
    if (a - b == 1) {
      if (a % 2 != 0) {
        cout << y << endl;
        return;
      }
    }
    cout << "-1\n";

    return;
  }

  int diff = b - a;
  int xdiff = diff * x;
  if (diff % 2 == 0) {
    cout << min(xdiff, ((diff * x) / 2) + ((diff * y) / 2)) << endl;
    return;
  } else {
    if (a % 2 != 0) {

      cout << min(xdiff, (diff / 2 + 1) * x + (diff / 2) * y) << endl;
      return;
    } else {
      cout << min(xdiff, (diff / 2) * x + (diff / 2 + 1) * y) << endl;
      return;
    }
  }
}

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