/** 
 *    author:  pankaj_m05
 *    created: 06.02.2021 17:35:00
**/ 
#include<bits/stdc++.h>

using namespace std;

int main(void) {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  int v, t, s, d;
  cin >> v >> t >> s >> d;
  if (d < v * t || d > v * s) {
    cout << "Yes";
  } else {
    cout << "No";
  }
  return 0;
}        
#include <iostream>
#include <fstream>
#include <set>
#include <map>
#include <string>
#include <vector>
#include <queue>
#include <deque>
#include <stack>
#include <functional>
#include <algorithm>
#include <climits>
#include <cmath>
#include <iomanip>
using namespace std;
#define ll long long int
#define rep(i,n) for( int i = 0; i < n; i++ )
#define rrep(i,n) for( int i = n; i >= 0; i-- )
#define REP(i,s,t) for( int i = s; i <= t; i++ )
#define RREP(i,s,t) for( int i = s; i >= t; i-- )
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define INF 2000000000
#define mod 1000000007
#define INF2 1000000000000000000


int main(void)
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    int V, S, T, D;
    cin >> V >> S >> T >> D;
    int L = V * S;
    int R = V * T;
    if(L <= D && D <= R) {
        cout << "No" << endl;
    } else {
        cout << "Yes" << endl;
    }

    return 0;
}
