#include <algorithm>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <string>
#include <vector>

using namespace std;
typedef long long int ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;

#define rep(i,x) for(int i=0;i<(int)(x);++i)
#define reps(i,x) for(int i=1;i<=(int)(x);++i)
#define rrep(i,x) for(int i=((int)(x)-1);i>=0;i--)
#define pb push_back

#define FOR(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)
#define ALLOF(c) (c).begin(), (c).end()

#define DEBUG

#ifdef DEBUG 
#define D(x) x
#else 
#define D(x)
#endif

using namespace std;

void print_map(map<string, int> m) {
    FOR(itr, m) {
        std::cout << itr->first << " => " << itr->second << endl;
    }
}

void print_vector(vector<int> v) {
    FOR(itr, v) {
        std::cout << *itr << ", ";
    }
    cout << endl;
}

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

    int h, w, n, k;
    cin >> h >> w >> n >> k;
    int c = h * w - 1;
    cout << ((c % n == k - 1) ? "YES" : "NO") << endl;

    return 0;
}