#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using T = tuple<ll, ll, int>;
ll Y;
bool operator<(T& t1, T& t2) {
  return (Y - get<0>(t1)) * get<1>(t2) < (Y - get<0>(t2)) * get<1>(t1);
}
int main() {
  int N, M, P;
  cin >> N >> M >> P >> Y;
  vector<vector<pair<int, ll>>> G(N);
  for (int i = 0; i < M; i++) {
    int a, b, c;
    cin >> a >> b >> c;
    a--, b--;
    G[a].push_back({b, c});
    G[b].push_back({a, c});
  }
  map<int, ll> mp;
  for (int i = 0; i < P; i++) {
    ll d, e;
    cin >> d >> e;
    mp[d - 1] = e;
  }
  // (cost, e, to)を乗せる
  // cost/eの最大化
  // (Y-cost1)*e2<(Y-cost2)*e1
  ll ans{};
  priority_queue<T> que;
  if (!mp.contains(0))
    que.push({0, 1000000000, 0});
  else
    que.push({0, mp[0], 0});
  vector<ll> dist(N);
  while (!que.empty()) {
    auto [c, e, now] = que.top();
    que.pop();
    ans = max(ans, (Y - c) / e);
    for (auto&& [p, c2] : G[now]) {
      if (c + c2 > Y) continue;
      auto e2 = e;
      if (mp.contains(p)) e2 = mp[p];
      T t2{c + c2, e2, p};
      if (dist[p] > (c + c2) / e2) continue;
      que.push({t2});
    }
  }
  cout << ans << endl;
}