#include <algorithm>
#include <iostream>
#include <iomanip>
#include <limits.h>
#include <map>
#include <math.h>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <string>
#include <utility>
#include <vector>
#include <stack>
#include <complex>

using namespace std;
#define rep(i, n) for (int i = 0; i < n; i++)
#define rep1(i, n) for (int i = 1; i < n + 1; i++)
#define rev(i, n) for (int i = n - 1; i >= 0; i--)
#define all(A) A.begin(), A.end()
#define itr(A, l, r) A.begin() + l, A.begin() + r
#define debug(var) cout << #var << " = " << var << endl;
typedef long long ll;

template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p)
{
    os << "(" << p.first << "," << p.second << ")";
    return os;
}

template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p)
{
    is >> p.first >> p.second;
    return is;
}

template <typename T>
ostream &operator<<(ostream &os, const vector<T> &v)
{
    for (int i = 0; i < (int)v.size(); i++)
    {
        os << v[i] << (i + 1 != (int)v.size() ? " " : "");
    }
    return os;
}

template <typename T>
ostream &operator<<(ostream &os, const vector<vector<T>> &v)
{
    for (int i = 0; i < (int)v.size(); i++)
    {
        os << v[i] << endl;
    }
    return os;
}

template <typename T>
ostream &operator<<(ostream &os, const vector<vector<vector<T>>> &v)
{
    int n = v.size();
    int m = v[0].size();
    int p = v[0][0].size();
    rep(k, p)
    {
        os << "k = " << k << endl;
        rep(i, n)
        {
            rep(j, m)
            {
                os << v[i][j][k];
                if (j < m - 1)
                {
                    os << " ";
                }
                else
                {
                    os << endl;
                }
            }
        }
    }
    return os;
}

template <typename T>
istream &operator>>(istream &is, vector<T> &v)
{
    for (T &in : v)
        is >> in;
    return is;
}

template <typename T, typename S>
ostream &operator<<(ostream &os, map<T, S> &mp)
{
    for (auto &[key, val] : mp)
    {
        os << key << ":" << val << " ";
    }
    cout << endl;
    return os;
}

template <typename T>
ostream &operator<<(ostream &os, set<T> st)
{
    auto itr = st.begin();
    for (int i = 0; i < (int)st.size(); i++)
    {
        os << *itr << (i + 1 != (int)st.size() ? " " : "");
        itr++;
    }
    return os;
}

template <typename T>
ostream &operator<<(ostream &os, multiset<T> st)
{
    auto itr = st.begin();
    for (int i = 0; i < (int)st.size(); i++)
    {
        os << *itr << (i + 1 != (int)st.size() ? " " : "");
        itr++;
    }
    return os;
}

template <typename T>
ostream &operator<<(ostream &os, queue<T> q)
{
    while (q.size())
    {
        os << q.front() << " ";
        q.pop();
    }
    return os;
}

template <typename T>
ostream &operator<<(ostream &os, deque<T> q)
{
    while (q.size())
    {
        os << q.front() << " ";
        q.pop_front();
    }
    return os;
}

template <typename T>
ostream &operator<<(ostream &os, stack<T> st)
{
    while (st.size())
    {
        os << st.top() << " ";
        st.pop();
    }
    return os;
}

template <typename T>
ostream &operator<<(ostream &os, priority_queue<T> pq)
{
    while (pq.size())
    {
        os << pq.top() << " ";
        pq.pop();
    }
    return os;
}

template <typename T>
ostream &operator<<(ostream &os, priority_queue<T, vector<T>, greater<T>> mpq)
{
    while (mpq.size())
    {
        os << mpq.top() << " ";
        mpq.pop();
    }
    return os;
}
struct Edge {
    long long to;
    long long cost;
};
using Graph = vector<vector<Edge>>;
using P = pair<long, int>;
const long long INF = 1LL << 60;
void dijkstra(const Graph &G, int s, vector<long long> &dis) {
    int N = G.size();
    dis.resize(N, INF);
    priority_queue<P, vector<P>, greater<P>> pq;  // 「仮の最短距離, 頂点」が小さい順に並ぶ
    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) {  // 最短距離で無ければ無視
            continue;
        }
        for (auto &e : G[v]) {
            if (dis[e.to] > dis[v] + e.cost) {  // 最短距離候補なら priority_queue に追加
                dis[e.to] = dis[v] + e.cost;
                pq.emplace(dis[e.to], e.to);
            }
        }
    }
}
int main(){
    ll n,m,p,y;
    cin >> n >> m >> p >> y;
    Graph to(n);
    rep(i,m){
        ll a,b,c;
        cin >> a >> b >> c;
        a--;
        b--;
        to[a].push_back({b,c});
        to[b].push_back({a,c});
    }
    vector<ll> dist;
    dijkstra(to,0,dist);
    // debug(dist);
    ll ans = 0;
    rep(i,p){
        ll d,e;
        cin >> d >> e;
        d--;
        ll money = max(0LL,y-dist[d]);
        // debug(money);
        auto chmax = [](auto &a, auto b)
        { a = max(a, b); };
        chmax(ans,money/e);
    }
    cout << ans << endl;
}