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

struct ios
{
    inline char read()
    {
        static const int IN_LEN = 1e6 + 10;
        static char buf[IN_LEN], *s, *t;
        return (s == t) && (t = (s = buf) + fread(buf, 1, IN_LEN, stdin)), s == t ? -1 : *s++;
    }
    template <typename _Tp>
    inline ios &operator>>(_Tp &x)
    {
        static char c11, boo;
        for (c11 = read(), boo = 0; !isdigit(c11); c11 = read())
        {
            if (c11 == -1)
                return *this;
            boo |= c11 == '-';
        }
        for (x = 0; isdigit(c11); c11 = read())
            x = x * 10 + (c11 ^ '0');
        boo && (x = -x);
        return *this;
    }
} io;

namespace IO
{
    template <typename T>
    inline void w(T x)
    {
        if (x > 9)
            w(x / 10);
        putchar(x % 10 + 48);
    }
    template <typename T>
    inline void write(T x, char c)
    {
        if (x < 0)
            putchar('-'), x = -x;
        w(x);
        putchar(c);
    }
    template <typename T>
    inline void read(T &x)
    {
        x = 0;
        T f = 1;
        char c = getchar();
        for (; !isdigit(c); c = getchar())
            if (c == '-')
                f = -1;
        for (; isdigit(c); c = getchar())
            x = (x << 1) + (x << 3) + (c ^ 48);
        x *= f;
    }
} // namespace IO

#define int long long
#define x first
#define y second
#define PII pair<int,int>
#define LL long long
#define pb push_back
#define pqp priority_queue<PII,vector<PII>,greater<PII>>
#define pqi priority_queue<int,vector<int>,greater<int>>
#define PSS pair<string,string>
#define in insert
#define line inline
#define sort(s) sort(s.begin(),s.end());
#define reverse(s) reverse(s.begin(),s.end());
#define max_(s) *max_element(s.begin(), s.end())
#define V vector<int>
#define VV vector<V>
#define IOS ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);

int dx[]=  {0,-1,0,1,0,0},dy[]= {-1,0,1,0,0,0},dz[] = {0,0,0,0,-1,1};

void solve()
{
    int v,t,s,d;
    io >> v >> t >> s >> d;
    int x1 = v * t, x2 = s * v;
    if(d >=x1 && d <= x2) puts("No");
    else puts("Yes");
}

signed main()
{
    // IOS
    // int _; cin >> _; while(_ --)
    solve();
}

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

int main() {
    int V,T,S,D;
    cin >> V >> T >> S >> D;
    cout << ((V*T <= D && V*S >= D)?"No":"Yes") << endl;
}