//#pragma GCC optimize("Ofast")
//#pragma GCC optimize "O3,omit-frame-pointer,inline"
#include <iostream> // cout, endl, cin
#include <string> // string, to_string, stoi
#include <vector> // vector
#include <algorithm> // min, max, swap, sort, reverse, lower_bound, upper_bound
#include <utility> // pair, make_pair
#include <tuple> // tuple, make_tuple
#include <cstdint> // int64_t, int*_t
#include <cstdio> // printf
#include <map> // map
#include <queue> // queue, priority_queue
#include <set> // set
#include <stack> // stack
#include <deque> // deque
#include <unordered_map> // unordered_map
#include <unordered_set> // unordered_set
#include <bitset> // bitset
#include <cctype> // isupper, islower, isdigit, toupper, tolower
#include <iomanip>//fixed,setprecision
#include <limits.h>//INT_MAX
#include <math.h>//M_PI
#include <random>
#include <regex> // 正規表現
#include <time.h>
#include <fstream>
#include <array>
#include <bit>
#include <chrono>
#include <span>
#include <cmath>
#include <complex>//複素数
//#include <bits/stdc++.h>
using namespace std;
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using ll = long long;
using ull = unsigned long long;

#define rep(i, n) for (ll i = 0; i < (ll)(n); i++)
const int dx[4] = {0, 1, 0, -1};
const int dy[4] = {1, 0, -1, 0};
random_device rnd;
mt19937 mt_gen(rnd());
int RandInt(int a, int b) {
    return a + mt_gen() % (b - a + 1);
}

struct edge{
    ll to,cost;
};
ll n,m,p,y;
vector<edge>to[202020];
ll ans_cnt=0;
ll goal=-1;
void dijkstra(ll s,  vector<ll>&d){
    priority_queue<pair<ll,ll>,vector<pair<ll,ll>>,greater<pair<ll,ll>>>pq;
    d[s]=0;
    pq.push({0,s});
    while(!pq.empty()){
        pair<ll,ll>p=pq.top();
        pq.pop();
        ll v=p.second;
        if(d[v]<p.first)continue;
        for(auto e:to[v]){
            if(d[e.to]>d[v]+e.cost){
                d[e.to]=d[v]+e.cost;
                pq.push({d[e.to],e.to});
            }
        }
    }
}
int main(){
    cin>>n>>m>>p>>y;
    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<pair<ll,ll>>shop(p);
    rep(i,p){
        cin>>shop[i].second>>shop[i].first;
        shop[i].second--;
    }
    sort(shop.begin(),shop.end());
    goal=shop[0].second;
    vector<ll>d(n,1e18);
    dijkstra(0,d);
    rep(i,p){
        if(y-d[shop[i].second]>0){
            ans_cnt=max(ans_cnt,(y-d[shop[i].second])/shop[i].first);
        }
    }
    cout<<ans_cnt<<endl;
    return 0;
}