#include <bits/stdc++.h>
using namespace std;
int main(){
  int V, T, S, D;
  cin >> V >> T >> S >> D;
  T *= V;
  S *= V;
  if(T <= D && D <= S) cout << "No" << endl;
  else cout << "Yes" << endl;
  return 0;
}
#pragma GCC optimize("Ofast")
#include<bits/stdc++.h>
 
#define fi first
#define se second
#define INF 0x3f3f3f3f
#define STD std::ios::sync_with_stdio(false);cin.tie(0);cout.tie(0)
#define FOR(i,a,b) for (int i(a); i < (b); i++)
#define REP(x, l, u) for(int x = l; x <= u; x++)
#define UN(v) sort((v).begin(),(v).end()),v.erase(unique(v.begin(),v.end()),v.end())
#define CL(a,b) memset(a,b,sizeof(a))
#define pb push_back
#define SORT(a) sort((a).begin(),(a).end())
#define mp(a, b) make_pair(a, b)
 
const int N = 1e5+10;
using namespace std;
 
typedef long long LL;
typedef pair<int, int> PII;
typedef vector<int> VI;
 
LL v, t, s, d;
 
int main() {
    STD;
    cin >> v >>t >>s >>d;
    t = v * t;
    s = s * v;
    if (d >= t && d <= s) cout << "No";
    else cout << "Yes";
    return 0;
}
