#include <bits/stdc++.h>
using namespace std;
int main() {
  double n, t, k, d;
  cin >> n >> t >> k >> d;
  int countNeed = ceil(n / k);
  int timeNeed = countNeed * t;
  int df = timeNeed - d;
  int tf = int(t);
  int res = 0;
  if (df > tf) {
    res = floor(df / tf);
  }
  int totalTime = (res * k) + ((countNeed - 1) * k);
  if (res != 0 && totalTime > countNeed) {
    cout << "YES" << endl;
  } else {
    cout << "NO" << endl;
  }
  return 0;
}
