/**
	author:  shobonvip
	created: 2025.01.25 23:16:58
**/

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

//* ATCODER
#include<atcoder/all>
using namespace atcoder;
typedef modint998244353 mint;
//*/

/* BOOST MULTIPRECISION
#include<boost/multiprecision/cpp_int.hpp>
using namespace boost::multiprecision;
//*/

typedef long long ll;

#define rep(i, s, n) for (int i = (int)(s); i < (int)(n); i++)
#define rrep(i, s, n) for (int i = (int)(n)-1; i >= (int)(s); i--)
#define all(v) v.begin(), v.end()

template <typename T> bool chmin(T &a, const T &b) {
	if (a <= b) return false;
	a = b;
	return true;
}

template <typename T> bool chmax(T &a, const T &b) {
	if (a >= b) return false;
	a = b;
	return true;
}

template <typename T> T max(vector<T> &a){
	assert(!a.empty());
	T ret = a[0];
	for (int i=0; i<(int)a.size(); i++) chmax(ret, a[i]);
	return ret;
}

template <typename T> T min(vector<T> &a){
	assert(!a.empty());
	T ret = a[0];
	for (int i=0; i<(int)a.size(); i++) chmin(ret, a[i]);
	return ret;
}

template <typename T> T sum(vector<T> &a){
	T ret = 0;
	for (int i=0; i<(int)a.size(); i++) ret += a[i];
	return ret;
}

int main(){
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	ll n,m,p,y;
	cin >> n >> m >> p >> y;
	vector ikeru(n, vector<pair<int,ll>>(0));
	rep(i,0,m){
		int a, b; cin >> a >> b;
		a--; b--;
		ll c; cin >> c;
		ikeru[a].push_back(pair(b, c));
		ikeru[b].push_back(pair(a, c));
	}

	vector<ll> d(n, 1e18);
	d[0] = 0;
	priority_queue<pair<ll,int>> pq;
	pq.push(pair(0, 0));
	while(!pq.empty()) {
		auto [tmp, i] = pq.top();
		pq.pop();
		tmp = -tmp;
		if (tmp > d[i]) continue;
		for (auto [j, c]: ikeru[i]) {
			if (chmin(d[j], d[i] + c)) {
				pq.push(pair(-d[j],j));
			}
		} 
	}

	ll ans = 0;
	rep(i,0,p) {
		int x;
		ll e; cin >> x >> e;
		x--; 
		ll left = max(0LL, y - d[x]);
		chmax(ans, left / e);
	}
	cout << ans << '\n';
}