#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
typedef long long LL;

int main(){
    LL H, W, N, K;
    cin >> H >> W >> N >> K;
    if((H*W-1) % N == K-1){
        cout << "YES" << endl;
    }else{
        cout << "NO" << endl;
    }
    return 0;
}