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

/*
ショトカ登録
cin cout endl return void
int double string char
pair make_pair first second tuple make_tuple get
sort erase swap map set insert
stoi to_string
auto bool true false
vector queue priority_queue front pop_frpnt
size substr push_back pop_back push pop top
if for while break continue next_permutation
__builtin_popcount 
*/

//無限
int inf = 1020304050;
int64_t INF = 1020304050607080900;

//モッド
int64_t mod = 998244353;

//all マクロ
#define all(v) v.begin(), v.end()

//long long
typedef long long lint;


int main () {
    //少数以下
    cout << fixed << setprecision(15);

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