#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); i++)
using namespace std;
using ll = long long;

int main()
{

    int v, t, s, d;
    cin >> v >> t >> s >> d;

    if (t * v <= d && d <= s * v)
    {
        cout << "No" << endl;
    }
    else
    {
        cout << "Yes" << endl;
    }

    return 0;
}
#include<bits/stdc++.h>

using namespace std;

#define ll long long int
#define get(a) ll a ; cin >> a ;
#define show(a) cout << a << endl;

int main()
{
	ios_base::sync_with_stdio(false);
    cin.tie(NULL);

	get(v);
	get(t);
	get(s);
	get(d);

	float time = d/float(v);

	if ( time < t or time > s)
		cout << "Yes"<< endl;
	else
		cout<< "No" << endl;

	return 0;
}