#include <bits/stdc++.h>
using namespace std;
static const int INF = 0x3f3f3f3f;
static const long long INFL = 0x3f3f3f3f3f3f3f3fLL;
template <typename T, typename U>
static void amin(T &x, U y) {
  if (y < x) x = y;
}
template <typename T, typename U>
static void amax(T &x, U y) {
  if (x < y) x = y;
}
int main() {
  int n;
  int t;
  int k;
  int d;
  while (~scanf("%d%d%d%d", &n, &t, &k, &d)) {
    int m = (n + k - 1) / k;
    bool ans = false;
    for (int(i) = (int)(0); (i) <= (int)(m); ++(i))
      ans |= m * t > max(i * t, d + (m - i) * t);
    puts(ans ? "YES" : "NO");
  }
  return 0;
}
