#include <bits/stdc++.h>
using namespace std;

using lint = long long int;
using pint = pair<int, int>;
using plint = pair<lint, lint>;
#define ALL(x) (x).begin(), (x).end()
#define SZ(x) ((lint)(x).size())
#define POW2(n) (1LL << (n))
#define FOR(i, begin, end) for (int i = (begin), i##_end_ = (end); i < i##_end_; i++)
#define IFOR(i, begin, end) for (int i = (end)-1, i##_begin_ = (begin); i >= i##_begin_; i--)
#define REP(i, n) FOR(i, 0, n)
#define IREP(i, n) IFOR(i, 0, n)

int main()
{
    lint v,t,s,d;
    cin >> v >> t >> s >> d;
    cout << (!(v*t<=d&&d<=v*s)?"Yes":"No") << "\n";
    return 0;
}
#include <bits/stdc++.h>
using namespace std;

int V, T, S, D;

int main() {
	scanf("%d%d%d%d", &V, &T, &S, &D);
	printf(D < V * T || D > V * S ? "Yes" : "No");
	return 0;
}