#include<bits/stdc++.h>
using namespace std;
using ll = long long;
using PAIR = pair<int, int>;
using PAIRLL = pair<ll,ll>;
using vi = vector<int>;
using vll = vector<ll>;
using vvi = vector<vi>;
using vs = vector<string>;
#define rep(i,n) for(int i=0;i<(n);i++)
#define INF 100000000
#define REP(i,n) for(ll i=0;i<ll(n);i++)
#define REPD(i,n) for(ll i=n-1;i>=0;i--)
#define FOR(i,a,b) for(ll i=a;i<=ll(b);i++)
#define FORD(i,a,b) for(ll i=a;i>=ll(b);i--)
#define FORA(i,I) for(const auto& i:I)
//x:コンテナ
#define ALL(x) x.begin(),x.end() 
#define SIZE(x) ll(x.size()) 
//定数
#define INF32 2147483647 //2.147483647×10^{9}:32bit整数のinf
#define INF64 9223372036854775807 //9.223372036854775807×10^{18}:64bit整数のinf
#define MOD 1000000007 //問題による
#define F first
#define S second
//出力(空白区切りで昇順に)
#define coutALL(x) for(auto i=x.begin();i!=--x.end();i++)cout<<*i<<" ";cout<<*--x.end()<<endl
#define coutALLn(x) for(auto i=x.begin();i!=--x.end();i++)cout<<*i<<endl;cout<<*--x.end()<<endl
ll myceil(ll a,ll b){return (a+(b-1))/b;}
ll myfloor(ll a,ll b){return a/b;}

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