#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using mint = modint998244353;
//using mint = modint1000000007;
using ll = long long;
ll INF = 2e18;
template<typename T> using vc = vector<T>;
template<typename T> using vv = vc<vc<T>>;
using vi = vc<int>; using vvi = vv<int>;
using vl = vc<ll>; using vvl = vv<ll>;
using vs = vc<string>; using vvs = vv<string>;
using vb = vc<bool>; using vvb = vv<bool>;
using vmint = vc<mint>; using vvmint = vv<mint>;
#define rep(i,n) for(ll i=0; i<(n); i++)
#define drep(i,n) for(ll i=(n)-1; i>=0; i--)
#define rrep(i,n) for(ll i=1; i<=(n); i++)
#define nfor(i,a,b) for(ll i=a;i<b;i++)
#define dfor(i,a,b) for(ll i=(a)-1; i>=(b); i--)
template<class T>istream& operator>>(istream& i, vc<T>& v) {rep(j,(ll) size(v))i >> v[j]; return i; }
#define nall(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define chmax(x,y) x = max(x,y)
#define chmin(x,y) x = min(x,y)
#define YES cout<<"Yes"<<endl
#define NO cout<<"No"<<endl
#define YN {YES;}else{NO;}
#define ERROR cout<<-1<<endl
void print(long double x){ printf("%.20Lf\n",x);}
#define vc_cout(v){ll n = size(v);rep(i,n)cout<<v[i]<<endl;}
#define vv_cout(v){ll n = size(v);rep(i,n){rep(j,size(v[i])){cout<<' '<<v[i][j];}cout<<endl;}}

using Pair = pair<ll,ll>;

int main(){
    ll N, M, P, Y;
    cin >> N >> M >> P >> Y;
    vv<Pair> edge(N);
    rep(i,M) {
        ll a, b, c;
        cin >> a >> b >> c;
        a--,b--;
        edge[a].push_back({b,c});
        edge[b].push_back({a,c});
    }
    vl cost(N);
    rep(i,P) {
        ll d, e;
        cin >> d >> e;
        cost[d-1] = e;
    }
    vl distances(N,INF);
    distances[0] = 0;
    priority_queue<Pair,vc<Pair>,greater<Pair>> pq;
    pq.push({0,0});
    while(pq.size()>0) {
        ll distance = pq.top().first;
        ll from = pq.top().second;
        pq.pop();
        if (distance != distances[from]) continue;
        for (auto e : edge[from]) {
            ll d = e.second + distance;
            if (d<distances[e.first]) {
                pq.push({d,e.first});
                distances[e.first] = d;
            }
        }
    }
    ll ans = 0;
    rep(i,N) {
        if (cost[i] == 0) continue;
        ll cnt = (Y-distances[i])/cost[i];
        chmax(ans,cnt);
    }
    cout << ans << endl;
}