#include <bits/stdc++.h>
using namespace std;
int main() {
  int cakesneeded, timek, k, timeforoven;
  cin >> cakesneeded >> timek >> k >> timeforoven;
  int cyclesneeded = cakesneeded / k;
  if (cakesneeded % k != 0) {
    cyclesneeded++;
  }
  int totaltime = cyclesneeded * timek;
  int partialtime = totaltime - timek;
  if (partialtime > timeforoven) {
    cout << "YES";
  } else {
    cout << "NO";
  }
}
