/* -*- coding: utf-8 -*-
 *
 * 3013.cc:  No.3013 繝上メ繝槭く雋ｷ縺・弌莠ｺ - yukicoder
 */

#include<cstdio>
#include<vector>
#include<queue>
#include<algorithm>
#include<utility>

using namespace std;

/* constant */

const int MAX_N = 200000;

/* typedef */

using ll = long long;
using pii = pair<int,int>;
using pli = pair<ll,int>;
using vpii = vector<pii>;

/* global variables */

vpii nbrs[MAX_N];
int es[MAX_N];
ll ds[MAX_N];

/* subroutines */

/* main */

int main() {
  int n, m, p;
  ll y;
  scanf("%d%d%d%lld", &n, &m, &p, &y);

  for (int i = 0; i < m; i++) {
    int u, v, w;
    scanf("%d%d%d", &u, &v, &w);
    u--, v--;
    nbrs[u].push_back({v, w});
    nbrs[v].push_back({u, w});
  }
  
  for (int i = 0; i < p; i++) {
    int di, ei;
    scanf("%d%d", &di, &ei), di--;
    es[di] = ei;
  }

  ds[0] = y;
  priority_queue<pli> q;
  q.push({y, 0});

  ll maxh = 0;
  while (! q.empty()) {
    auto [ud, u] = q.top(); q.pop();
    if (ds[u] != ud) continue;

    if (es[u] > 0) maxh = max(maxh, ud / es[u]);

    for (auto [v, w]: nbrs[u]) {
      int vd = ud - w;
      if (ds[v] < vd) ds[v] = vd, q.push({vd, v});
    }
  }

  printf("%lld\n", maxh);
  
  return 0;
}