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

int main(){
    int v,t,s,d;
    cin >> v >> t >> s >> d;
    if(v*t <= d && v*s >= d){
        cout << "No" << endl;
    }else{
        cout << "Yes" << endl;
    }
}
 /* Code by Brother_mohit */
     
    #pragma GCC optimize("Ofast")
    #pragma GCC target("avx,avx2,fma")
    #include<bits/stdc++.h>
    #define fastio  ios_base::sync_with_stdio(0); cin.tie(0);
    #define tc int tt; cin>>tt; for(int t=1; t<=tt; t++) 
    #define ll long long
    #define  um unordered_map
    #define us unordered_set
    #define pq priority_queue
    #define mod 1000000007
    #define endl  "\n"
    using namespace std; 

     // main code and functions *******************->\
  


     
    int main()
    {
      fastio
     
      // tc
      {
        
    //   int n ; 
    //   cin>>n;
       
    //     vector<int> a(n);
        
    //     for(int i=0; i<n; i++)
    //      {
    //           cin>>a[i];
    //      }
         
         
         int v,t,s,d;
         cin>>v>>t>>s>>d;
         
         int d1= v*t;
          int d2= v*s;
          
          if(d<d1||d>d2)
           cout<<"Yes"<<endl;
           else cout<<"No"<<endl;
     
       
       }
         return 0; 
    }