#include <bits/stdc++.h>
using namespace std;
int main() {
  long long N, T, K, D;
  cin >> N >> T >> K >> D;
  long long time1 = ceil((float)N / K);
  time1 *= T;
  long long start2 = (ceil((float)D / T)) * T;
  long long cakes_left = N - ((ceil((float)D / T)) * K);
  long long time2 = start2;
  long long i, j;
  i = 1;
  j = 1;
  while (cakes_left > 0) {
    cakes_left -= K;
    time2 = D + (i * T);
    i++;
    if (cakes_left > 0) {
      cakes_left -= K;
      time2 = start2 + (j * T);
      j++;
    } else
      break;
  }
  if (time2 < time1)
    cout << "YES";
  else
    cout << "NO";
  return 0;
}
