#include <cstdio>
#include <cmath>
#include <vector>
using namespace std;

int main(){
	using std::vector;
	float H, W, N, K;
	float size;
	fscanf(stdin, "%f %f %f %f", &H, &W, &N, &K);
	size = (H * W) - 1;
	size = fmod(size, N);
	if ((int)size == (int)(K-1)) {
		printf("YES\n");
	}
	else {
		printf("NO\n");
	}
	return 0;
}