/*
 * A.cpp
 *
 *  Created on: Feb 6, 2021
 *      Author: jrantor
 */



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


int main(){

double v,t,s,d;

cin >> v >> t >> s >> d;

double x = d/v;

if(x >= t && x <= s){
 cout << "No";
}

else cout << "Yes";


 return 0;
}


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

using ll  = long long;
using ull = unsigned ll;
using ld  = long double;
using pi  = pair<int, int>;
using pii = pair<ll, ll>;
using vi  = vector<int>;
using vii = vector<ll>;
using vip = vector<pair<int, int>>;

const int MAX_N = 1e5 + 1;
const ll mod = 1e9 + 7;
const ll INF = 1e9;

#define PI           3.141592653589793238
#define fir          first
#define sec          second
#define all(x)       (x).begin(),(x).end()
#define rall(x)      (x).rbegin(),(x).rend()
#define pb           push_back
#define mp           make_pair
#define loop(i,n,x)  for(int i=n;i<x;i++)
#define sz(a)        ((int) (a).size())
#define For(n,x)     for(int i=1;i<=x;i++)
#define ite          iterator it
#define fast         ios_base::sync_with_stdio(false);cin.tie(NULL);
#define test         ll t;cin>>t; while(t--){solve();}

void oj()
{


#ifndef ONLINE_JUDGE
	freopen("input.txt", "r", stdin);
	freopen("output.txt", "w", stdout);
#endif

}

bool isPowerOfTwo(ll n)
{
	if (n == 0)
		return false;

	return (ceil(log2(n)) == floor(log2(n)));
}

void solve()
{
	float v, t, s, d;
	cin >> v >> t >> s >> d;
	float result = d / v;
	if (result >= t && result <= s)
		cout << "No" << endl;
	else
		cout << "Yes" << endl;



}

int main()
{

	ios_base::sync_with_stdio(false); cin.tie(NULL);

	oj();

	//test;
	solve();


}

