#include <cstdio>
#include <iostream>
#include <cmath>
#include <algorithm>
#include <string>
#include <cstring>
using namespace std;
#define _ ios::sync_with_stdio(false),cin.tie(0)

const int MAXN = 5010;
const int INF = 0xfffffff;
typedef long long ll;

int t;
ll x,y,p,q;

int main()
{
    cin>>t;
    while(t--)
    {
        cin>>x>>y>>p>>q;
        ll l=0,r=1e9,ans=-1;
        while(l<=r)
        {
            ll mid=(l+r)>>1;
            ll a=mid*p-x,b=mid*q-y;
            if(a>=0&&b>=0&&a<=b)
            {
                ans=mid;
                r=mid-1;
            }
            else 
                l=mid+1;
        }
        if(ans==-1)
            cout<<-1<<endl;
        else
            cout<<ans*q-y<<endl;
    }
    return 0;
}