#include <bits/stdc++.h>

//#include <atcoder/all>
using namespace std;

#define lli long long int
#define REP(i, s, n) for (lli i = s; i < n; i++)
#define INF (1LL << 62)
#define mp(a, b) make_pair(a, b)
#define SORT(V) sort(V.begin(), V.end())
#define PI (3.141592653589794)
#define TO_STRING(VariableName) #VariableName
#define LOG1(x)                                                                \
  if (DEBUG)                                                                   \
    cout << TO_STRING(x) << "=" << x << " " << endl;
#define LOG2(x, y)                                                             \
  if (DEBUG)                                                                   \
    cout << TO_STRING(x) << "=" << x << " " << TO_STRING(y) << "=" << y << endl;
#define LOG3(x, y, z)                                                          \
  if (DEBUG)                                                                   \
    cout << TO_STRING(x) << "=" << x << " " << TO_STRING(y) << "=" << y << " " \
         << TO_STRING(z) << "=" << z << endl;
#define LOG4(w, x, y, z)                                                       \
  if (DEBUG)                                                                   \
    cout << TO_STRING(w) << "=" << w << " " << TO_STRING(x) << "=" << x << " " \
         << TO_STRING(y) << "=" << y << " " << TO_STRING(z) << "=" << z        \
         << endl;
#define LOG5(w, x, y, z, a)                                                    \
  if (DEBUG)                                                                   \
    cout << TO_STRING(w) << "=" << w << " " << TO_STRING(x) << "=" << x << " " \
         << TO_STRING(y) << "=" << y << " " << TO_STRING(z) << "=" << z << " " \
         << TO_STRING(a) << "=" << a << endl;
#define LOG6(w, x, y, z, a, b)                                                 \
  if (DEBUG)                                                                   \
    cout << TO_STRING(w) << "=" << w << " " << TO_STRING(x) << "=" << x << " " \
         << TO_STRING(y) << "=" << y << " " << TO_STRING(z) << "=" << z << " " \
         << TO_STRING(a) << "=" << a << " " << TO_STRING(b) << "=" << b        \
         << endl;

#define overload6(a, b, c, d, e, f, g, ...) g
#define LOG(...)                                                               \
  overload6(__VA_ARGS__, LOG6, LOG5, LOG4, LOG3, LOG2, LOG1)(__VA_ARGS__)

template <class T> bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}

mt19937 engine;
std::chrono::system_clock::time_point start, endTime;

#define DEBUG 0

/* dijkstra(G,s,dis,prev)
    入力：グラフ G, 開始点 s, 距離を格納する dis, 最短経路の前の点を記録するprev
    計算量：O(|E|log|V|)
    副作用：dis, prevが書き換えられる
*/
struct Edge {
  long long to;
  long long cost;
};
using Graph = vector<vector<Edge>>;
using P = pair<long, int>;
const long long INFL = 1LL << 60;

void dijkstra(const Graph &G, int s, vector<long long> &dis,
              vector<int> &prev) {
  int N = G.size();
  dis.resize(N, INFL);
  prev.resize(N, -1); // 初期化
  priority_queue<P, vector<P>, greater<P>> pq;
  dis[s] = 0;
  pq.emplace(dis[s], s);
  while (!pq.empty()) {
    P p = pq.top();
    pq.pop();
    int v = p.second;
    if (dis[v] < p.first) {
      continue;
    }
    for (auto &e : G[v]) {
      if (dis[e.to] > dis[v] + e.cost) {
        dis[e.to] = dis[v] + e.cost;
        prev[e.to] = v; // 頂点 v を通って e.to にたどり着いた
        pq.emplace(dis[e.to], e.to);
      }
    }
  }
}

/* get_path(prev, t)
    入力：dijkstra で得た prev, ゴール t
    出力： t への最短路のパス
*/
vector<int> get_path(const vector<int> &prev, int t) {
  vector<int> path;
  for (int cur = t; cur != -1; cur = prev[cur]) {
    path.push_back(cur);
  }
  reverse(path.begin(), path.end()); // 逆順なのでひっくり返す
  return path;
}

void solve() {
  //  write your code here

  //買う場所決めてそこで無限購入するくらいしかわからない。
  //ダイクストラで街までどれくらいかかるかで見る

  lli N, M, P, Y;
  cin >> N >> M >> P >> Y;

  Graph graph(N);

  REP(i, 0, M) {
    lli a, b, c;
    cin >> a >> b >> c;
    a--;
    b--;
    graph[a].push_back({b, c});
    graph[b].push_back({a, c});
  }

  vector<lli> D(P), E(P);
  REP(i, 0, P) {
    cin >> D[i] >> E[i];
    D[i]--;
  }

  vector<lli> dis;
  vector<int> prev;
  dijkstra(graph, 0, dis, prev);

  lli ans = 0;
  REP(i, 0, P) {
    lli les = Y - dis[D[i]];
    if (les <= 0) {
      continue;
    }
    chmax(ans, les / E[i]);
  }
  cout << ans << endl;
}

// Generated by 2.13.0 https://github.com/kyuridenamida/atcoder-tools  (tips:
// You use the default template now. You can remove this line by using your
// custom template)
int main() {
  // Failed to predict input format

  lli N = 1;
  // cin >> N;
  while (N--)
    solve();

  return 0;
}