import java.util.*;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int T = sc.nextInt();
        while(T-- > 0) {
            long n = sc.nextLong(), d = sc.nextLong(), x = sc.nextLong(), y = sc.nextLong();
            long[] a = new long[1];
            long[] b = new long[1];
            long gcd = exgcd(n, d, a, b);
            if((y - x) % gcd != 0) {
                System.out.println("Impossible");
            }else {
                b[0] *= (y - x) / gcd;
                n /= gcd;
                System.out.println((b[0] % n + n) % n);
            }
        }
        sc.close();
    }

    private static long exgcd(long a, long b, long[] x, long[] y) {
        if(b == 0) {
            x[0] = 1;
            y[0] = 0;
            return a;
        }
        long d = exgcd(b, a % b, y, x);
        y[0] -= a / b * x[0];
        return d;
    }
}