#pragma region shig_templates

#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
typedef pair<int, int> pairI2;
typedef pair<LL, LL> pairLL2;
typedef vector<int> VI;
typedef vector<LL> VLL;
typedef vector<VI> VVI;
typedef vector<VVI> VVVI;
typedef vector<string> VS;
#define shig_for(i, a, b) for(int i = (a); i < (b); ++i)
#define shig_rep(i, n) shig_for(i, 0, n)
#define shig_forB(bit,a,b) for(int bit = (a); bit < (1<<(b)); ++bit)
#define shig_repB(bit,n) shig_forB(bit,0,n)
#define CINI(a) int (a); cin >> (a)
#define CINLL(a) LL (a); cin >> (a)
#define CINI2(a, b) int a, b; cin >> (a) >> (b)
#define CINLL2(a, b) LL a, b; cin >> (a) >> (b)
#define tmp int temp = 0
#define str string s

template<class T> void COUT(T&& t){ cout << t << endl; }
template<class T,class... Ts>
void COUT(T&& t,Ts&&... ts){ cout << t << " "; COUT(ts...); }
void CIN() {}
template<class T,class... Ts> void CIN(T&& t,Ts&&... ts){ cin >> t; CIN(ts...); }

#pragma endregion

int main(){

    int V, T, S, D;
    CIN(V, T, S, D);

    if(!(D >= T*V && D <= S*V)){
        COUT("Yes");
    }else{
        COUT("No");
    }

    //for(string& s : S) cin >> s;
    //cout << "Atcoder " << N << endl;
    //cout << fixed << setprecision(10) << ans << endl;

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

using namespace std;

const long long mod = 1e9 + 7;

long long add(long long a, long long b) {
    long long res = a + b;
    if (res >= mod) res -= mod;
    return res;
}

long long sub(long long a, long long b) {
    long long res = a - b + mod;
    if (res >= mod) res -= mod;
    return res;
}

long long mul(long long a, long long b) {
    return (((a % mod) * (b % mod)) % mod);
}

long long potolok(long long a, long long b){
    return (a + b - 1) / b;
}

long long gcd(long long x, long long y){
    if(y == 0){
        return x;
    }
    if(x == 0){
        return y;
    }
    if(x > y){
         return gcd(y, x % y);
    }
    else{
         return gcd(x, y % x);
    }
}

long long bin_pow(long long base, long long power, long long m){
    if(power == 1){
        return base;
    }
    if(power % 2 == 0){
        long long t = bin_pow(base, power / 2, m);
        return t * t % m;
    }
    else{
        return bin_pow(base, power - 1, m) * base % m;
    }
}

struct segtree{

    int razmer;

    vector<long long> tree;

    void Init(vector<long long> &v){
        int n = (int)v.size();
        razmer = 1;
        while(razmer < n) razmer *= 2;
        tree.assign(2 * razmer - 1, 0);
    }

    void build(vector<long long> &v, int x, int lx, int rx){
        if(rx - lx == 1){
            if(lx < (int)v.size()){
                tree[x] = v[lx];
            }
        }else{
            int m = (lx + rx) / 2;
            build(v, 2 * x + 1, lx, m);
            build(v, 2 * x + 2, m, rx);
            tree[x] = min(tree[2 * x + 1], tree[2 * x + 2]);
        }
    }

    void build(vector<long long> &v){
        build(v, 0, 0, razmer);
    }

    void cet(int i, int v, int x, int lx, int rx){
        if(rx - lx == 1){
            tree[x] = v;
            return;
        }else{
            int m = (lx + rx) / 2;
            if(i < m){
                cet(i, v, 2 * x + 1, lx, m);
            }else{
                cet(i, v, 2 * x + 2, m, rx);
            }
            tree[x] = min(tree[2 * x + 1], tree[2 * x + 2]);
        }
    }

    void cet(int i, int v){
        cet(i, v, 0, 0, razmer);
    }

    long long mn(int l, int r, int x, int lx, int rx){
        if(lx >= r || l >= rx){
            return mod;
        }
        if(lx >= l && rx <= r){
            return tree[x];
        }
        else if(min(rx, r) - max(lx, l) >= 0){
            int m = (lx + rx) / 2;
            long long s1 = mn(l, r, 2 * x + 1, lx, m);
            long long s2 = mn(l, r, 2 * x + 2, m, rx);
            return min(s1, s2);
        }
    }

    long long mn(int l, int r){
        return mn(l, r, 0, 0, razmer);
    }

};

void solve(){
    //std::freopen("stupid_rmq.in", "r", stdin);
    //std::freopen("stupid_rmq.out", "w", stdout);
    //ios_base::sync_with_stdio(false);
    //cin.tie(NULL);
    int V, T, S, D;
    cin >> V >> T >> S >> D;
    if(D >= V * T && D <= V * S){
        cout << "No" << endl;
    }else{
        cout << "Yes" << endl;
    }
}

int main(){
    int t;
    t = 1;
    //cin >> t;
    while(t--){
        solve();
    }
    return 0;
}
