#include<bits/stdc++.h>
using namespace std;
using ll = long long;

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }

#define FOR(i,a,b) for(ll i=(a);i<(b);++i)
#define ALL(v) (v).begin(), (v).end()
#define p(s) cout<<(s)<<endl
#define p2(s, t) cout << (s) << " " << (t) << endl
#define br() p("")
#define pn(s) cout << (#s) << " " << (s) << endl
#define p_yes() p("YES")
#define p_no() p("NO")

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);

    // input
    ll H, W, N, K;
    cin >> H >> W >> N >> K;

    ll a = H*W;
    ll b = a % N;
    if(b==0){
        ll last = N;
        if(K==last){
            p_yes();
        }else{
            p_no();
        }
    }else{
        if(b+1==K){
            p_yes();
        }else{
            p_no();
        }
    }
    
    return 0;
}