#include <iostream>
using namespace std;
int main()
{
    int v, t, s, d;
    cin >> v >> t >> s >> d;
    cout << (d < v*t || v*s < d ? "Yes" : "No") << endl;
    return 0;
}
#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define ld long double
#define Test int T;cin>>T;while(T--)
#define PI acos(-1)
#define endl "\n"
#define fx(x) fixed<<setprecision(x)
#define sz(s) (int)s.size()
#define all(v) (v).begin(),(v).end()
#define allr(v) (v).rbegin(),(v).rend()
#define mem(a,n) memset((a),n,sizeof (a))
#define INF 1e9
#define ii pair<int,int>
ll gcd(ll x,ll y){return(!y?x:gcd(y,x%y));}
ll lcm(ll x,ll y){return x/gcd(x,y)*y;}
void file(){
#ifndef ONLINE_JUDGE
    freopen("in.txt", "r", stdin);
    freopen("out.txt", "w", stdout);
#else
    //freopen("east.in", "r", stdin);
    //freopen("output.txt", "w", stdout);
#endif
}
void fast(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    file();
}
int dx[]= {-1,0,1,0};
int dy[]= {0,1,0,-1};
const double eps=1e-9;
const int mod=1e9+7;
const int N=1e5+7;
int main(){
    fast();
    int v,t,s,d;
    cin>>v>>t>>s>>d;
    if(d>=t*v&&d<=s*v){
        cout<<"No";
    }
    else cout<<"Yes";
}
