#include <bits/stdc++.h>
using namespace std;
template <typename T>
void keySort(vector<T> &arr, vector<int> &key) {
  int n = arr.size();
  if (key.size() != n) {
    throw runtime_error("Size of data array not equals to size of key array");
  }
  vector<T> ans(n);
  int ar[n];
  for (int i = 0; i < n; i++) {
    ar[key[i]] = i;
  }
  for (int i = 0; i < n; i++) {
    ans[i] = arr[ar[i]];
  }
  arr = ans;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> p) {
  os << '(' << p.first << ',' << p.second << ')';
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const vector<T> p) {
  os << "{ ";
  for (auto i : p) {
    os << i << ",";
  }
  os << "\b";
  os << "}";
  return os;
}
template <typename T1, typename T2>
istream &operator>>(istream &in, pair<T1, T2> &p) {
  in >> p.first >> p.second;
  return in;
}
void solve();
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t;
  cin >> t;
  while (t--) solve();
  return 0;
}
void solve() {
  long long x, y, a, b;
  cin >> x >> y >> a >> b;
  if (a == 0 && x != 0) {
    std::cout << -1 << endl;
    return;
  }
  if (a == 0 && x == 0) {
    std::cout << 0 << endl;
    return;
  }
  if (a == b && x != y) {
    std::cout << -1 << endl;
    return;
  }
  if (a == b && x == y) {
    std::cout << 0 << endl;
    return;
  }
  long long c = (x + a - 1) / a;
  long long ic = ((y - x) + (b - a) - 1) / (b - a);
  cout << (max(c, ic) * b - y) << endl;
}
