#include <algorithm>
#include <atcoder/all>
#include <bitset>
#include <cassert>
#include <cmath>
#include <ctime>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
using namespace std;
using namespace atcoder;
typedef long long ll;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repr(i, n) for (int i = (int)(n) - 1; i >= 0; i--)
#define repk(i, k, n) for (int i = k; i < (int)(n); i++)
#define all(v) v.begin(), v.end()
#define mod1 1000000007
#define mod2 998244353
#define mod3 100000007
#define vi vector<int>
#define vs vector<string>
#define vc vector<char>
#define vl vector<ll>
#define vb vector<bool>
#define vvi vector<vector<int>>
#define vvc vector<vector<char>>
#define vvl vector<vector<ll>>
#define vvb vector<vector<bool>>
#define vvvi vector<vector<vector<int>>>
#define vvvl vector<vector<vector<ll>>>
#define pii pair<int, int>
#define pil pair<int, ll>
#define pli pair<ll, int>
#define pll pair<ll, ll>
#define vpii vector<pair<int, int>>
#define vpll vector<pair<ll, ll>>
#define vvpii vector<vector<pair<int, int>>>
#define vvpll vector<vector<pair<ll, ll>>>

using mint = modint998244353;

template <typename T>
void debug(T e) {
    cerr << e << endl;
}

template <typename T>
void debug(vector<T> &v) {
    rep(i, v.size()) { cerr << v[i] << " "; }
    cerr << endl;
}

template <typename T>
void debug(vector<vector<T>> &v) {
    rep(i, v.size()) {
        rep(j, v[i].size()) { cerr << v[i][j] << " "; }
        cerr << endl;
    }
}

template <typename T>
void debug(vector<pair<T, T>> &v) {
    rep(i, v.size()) { cerr << v[i].first << " " << v[i].second << endl; }
}

template <typename T>
void debug(set<T> &st) {
    for (auto itr = st.begin(); itr != st.end(); itr++) {
        cerr << *itr << " ";
    }
    cerr << endl;
}

template <typename T>
void debug(multiset<T> &ms) {
    for (auto itr = ms.begin(); itr != ms.end(); itr++) {
        cerr << *itr << " ";
    }
    cerr << endl;
}

template <typename T>
void debug(map<T, T> &mp) {
    for (auto itr = mp.begin(); itr != mp.end(); itr++) {
        cerr << itr->first << " " << itr->second << endl;
    }
}

void debug_out() { cerr << endl; }

template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
    cerr << H << " ";
    debug_out(T...);
}

void dijkstra(vvpll &graph, ll v, vl &dist) {
    priority_queue<pll, vpll, greater<pll>> pque;
    pque.push(pll(0, v));
    dist[v] = 0;
    while (pque.size()) {
        pll p = pque.top();
        pque.pop();
        if (dist[p.second] != p.first) {
            continue;
        }
        for (auto x : graph[p.second]) {
            if (dist[x.first] > dist[p.second] + x.second) {
                dist[x.first] = dist[p.second] + x.second;
                pque.push(pll(dist[x.first], x.first));
            }
        }
    }
}

int main() {
    ll N, M, P, Y;
    cin >> N >> M >> P >> Y;
    vector<ll> A(M);
    vector<ll> B(M);
    vector<ll> C(M);
    for (ll i = 0; i < M; i++) {
        cin >> A[i] >> B[i] >> C[i];
        A[i]--;
        B[i]--;
    }
    vector<ll> D(P);
    vector<ll> E(P);
    for (ll i = 0; i < P; i++) {
        cin >> D[i] >> E[i];
        D[i]--;
    }
    vector<vector<pll>> graph(N, vector<pll>(0));
    for (ll i = 0; i < M; i++) {
        graph[A[i]].push_back(make_pair(B[i], C[i]));
        graph[B[i]].push_back(make_pair(A[i], C[i]));
    }
    ll ans = 0;

    ll INF = 100000000000000023;
    vector<ll> dist(N, INF);
    dijkstra(graph, 0, dist);

    for (ll i = 0; i < P; i++) {
        if (dist[i] >= INF) {
            continue;
        }
        ans = max(ans, (Y - dist[D[i]]) / E[i]);
    }
    cout << ans << endl;
}