#include <bits/stdc++.h>
using namespace std;
 
int main(){
    int V, T, S, D;
    cin >> V >> T >> S >> D;

    string ans = "Yes";
    if(V*T <= D){
        if(V*S >= D){
            ans = "No";
        }
    }

    cout << ans << endl;
    return 0;
}
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef vector<ll> vl;
typedef vector<int> vi;
#define rep(i, a, b) for (int i = a; i < b; i++)
#define mapi map<int, int>
#define mapl map<ll, ll>
#define pb push_back
#define fast_cin()                    \
    ios_base::sync_with_stdio(false); \
    cin.tie(NULL);                    \
    cout.tie(NULL)
#define all(x) (x).begin(), (x).end()
#define tsolve(t) \
    int t;        \
    cin >> t;     \
    while (t--)   \
    {             \
        solve();  \
    }
void solve()
{
    double v, t, s, d;
    cin >> v >> t >> s >> d;
    double tt = d / v;
    //cout << tt << endl;
    if (t <= tt && tt <= s)
    {
        cout << "No";
        return;
    }
    cout << "Yes";
    return;
}
int main()
{
    fast_cin();
    // tsolve(t)
    solve();
    return 0;
}