#include<bits/stdc++.h>
using namespace std;
#define fastio ios_base::sync_with_stdio(false);cin.tie(NULL);
#define int long long
#define inf 1000000000
#define pii pair<int,int>
#define fi first
#define se second
signed main(){
    fastio
    int t;
    //cin>>t;
    t=1;
    while(t--){
        int v,t,s,d;
        cin>>v>>t>>s>>d;
        int l=v*t,r=v*s;
        if(d>=l && d<=r)cout<<"No\n";
        else cout<<"Yes\n";
    }
    return 0;
}
#include <bits/stdc++.h>
using namespace std;
#define int               long long
#define ull               unsigned long long
#define si(x)             int x;cin>>x;
#define w(x)              si(x); while(x--)
#define fo(i,n)           for(int i=0 ; i<n ; i++)
#define rea(a,n)          int a[n];fo(i,n){cin>>a[i];}
#define rea2d(a,n,m)      int a[n][m];fo(i,n){fo(j,m){cin>>a[i][j];}}
#define pra(a,n)          fo(i,n){cout<<a[i]<<" ";}
#define prv(a,n)          for (auto i = a.begin(); i != a.end(); ++i){cout<<*i<<" ";}
#define prg(a,n)          for (auto ir = a.rbegin(); ir != a.rend(); ++ir){cout<<*i<<" ";}  
#define pb                push_back
#define mkp               make_pair
#define F                 first
#define S                 second 
#define all(v)            v.begin(),v.end()
#define e                 endl
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<pii> vpii;
typedef vector<vi> vvi; 
typedef deque<pii> dpii;
typedef unordered_map<int,int> um;
// int max 2147483647
void solution()
{	
	si(v);si(t);si(s);si(d);
	int q=d/v;
	if(d>=v*t&&d<=v*s){cout<<"No";}
  	else cout<<"Yes";
}

int32_t main()
{
    ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
    // w(t){
    //     solution();
    // }
    solution();
    return 0;

}