#include<iostream>
#include<cstring>
 
using namespace std;
 
typedef long long ll;
int t;
ll x,y,p,q;
 
bool check(ll k){
    return k * q - y >= k * p - x && k * p - x >= 0;
}
 
int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    
    cin >> t;
    while(t --){
        ll l = 1,r = 0x7fffffff;
        cin >> x >> y >> p >> q;
        while(l < r){
            ll mid = l + (r - l) / 2;
            if(check(mid)) r = mid;
            else l = mid + 1;
        }
        printf("%lld\n", l == 0x7fffffff ? -1 : l * q - y);
        
    }
    return 0;
}