#include<iostream>
using namespace std;

long long h, w, n, k;

int main() {
	cin >> h >> w >> n >> k;
	int s = h * w + 1;
	s %= n;
	
	if ( s + 1 == k )
		cout << "YES" << endl;
	else 
		cout << "NO" << endl;
	return 0;
}