#include <iostream>
using namespace std;
int main(void){
    int v,t,s,d;
    cin >> v >> t >> s >> d;
    if(v*t <= d && v*s >= d){
        cout << "No" << endl;
    }
    else cout << "Yes" << endl;
}

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

 
#define ff              first
#define ss              second
#define int             long long
#define pb              push_back
#define mp              make_pair
#define pii             pair<int,int>
#define vi              vector<int>
#define mii             map<int,int>
#define pqb             priority_queue<int>
#define pqs             priority_queue<int,vi,greater<int> >
#define setbits(x)      __builtin_popcountll(x)
#define zrobits(x)      __builtin_ctzll(x)
#define mod             1000000007
#define inf             1e18
#define ps(x,y)         fixed<<setprecision(y)<<x
#define w(x)            int x; cin>>x; while(x--)


int32_t main()
{
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    //fsolve();	
  	int v,t,s,d;
  cin>>v>>t>>s>>d;
  //int g=d/v;
  
  
  if(d<v*t || d>v*s){
    cout<<"Yes"<<endl;
  }
  else{
    cout<<"No"<<endl;
  }
    return 0;
}