#include <bits/stdc++.h>
using namespace std;
const long double inf = 1e18 + 7;
const long double eps = 1e-18;
long long gcd(long long a, long long b) {
  while (a || b) a %= b, swap(a, b);
  return a + b;
}
int main() {
  ios_base::sync_with_stdio(0);
  long long t;
  cin >> t;
  for (long long tt = 0; tt < t; ++tt) {
    long long x, y, p, q;
    cin >> x >> y >> p >> q;
    long long ok = y - x;
    long long wa = y - ok;
    long long nok = q - p;
    long long nwa = q - nok;
    if (nok == nwa) {
      cout << abs(ok - wa) << endl;
      continue;
    }
    if (nok == 0) {
      if (ok != 0) {
        cout << -1 << endl;
        continue;
      } else {
        cout << 0 << endl;
        continue;
      }
    }
    if (nwa == 0) {
      if (wa != 0) {
        cout << -1 << endl;
        continue;
      } else {
        cout << 0 << endl;
        continue;
      }
    }
    if (nok < nwa) {
      long long xx = ok + nok - 1;
      xx /= nok;
      xx *= nok;
      long long l = -1;
      long long r = 1e10;
      while (l != r - 1) {
        long long m = (l + r) / 2ll;
        long long lx = xx + nok * m;
        long long rx = lx / nok * nwa;
        if (rx >= wa)
          r = m;
        else
          l = m;
      }
      long long lx = xx + nok * r;
      long long rx = lx / nok * nwa;
      long long ans = lx - ok;
      ans += rx - wa;
      cout << ans << endl;
    } else {
      long long xx = wa + nwa - 1;
      xx /= nwa;
      xx *= nwa;
      long long l = -1;
      long long r = 1e10;
      while (l != r - 1) {
        long long m = (l + r) / 2;
        long long lx = xx + m * nwa;
        long long rx = lx / nwa * nok;
        if (rx >= ok)
          r = m;
        else
          l = m;
      }
      long long lx = xx + r * nwa;
      long long rx = lx / nwa * nok;
      long long ans = (lx - wa) + (rx - ok);
      cout << ans << endl;
    }
  }
  return 0;
}
