#include <bits/stdc++.h>
using namespace std;
int main() {
  int iTask = 0, iMin = 0, iCakes = 0, iBuild = 0;
  cin >> iTask >> iMin >> iCakes >> iBuild;
  int iSets = iTask / iCakes;
  if ((iSets * iCakes) != iTask) {
    iSets++;
  }
  int iNumOven = 0, iNumBuildOven = iBuild;
  iNumOven = iSets * iMin;
  int iNumDone = (iBuild + iMin) / iMin;
  int iLeft = iTask - (iCakes * iNumDone);
  int iLeftSet = (iLeft / iCakes);
  if ((iLeftSet * iCakes) != iLeft) {
    iLeftSet++;
  }
  int i = 0;
  do {
    iLeft -= iCakes;
    i += iMin;
  } while (iLeft > 0);
  iNumBuildOven += i;
  if (iNumOven > iNumBuildOven) {
    cout << "YES";
  } else {
    cout << "NO";
  }
}
