#include<bits/stdc++.h>
typedef long long ll;
using namespace std;
int INF = 1e9;
int MOD = 1e9+7;
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; 
}