#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define ALL(v) v.begin(),v.end()
typedef long long ll;

#include<bits/stdc++.h>
using namespace std;
template<class T>using V=vector<T>;
template<class T>using VV=V<V<T>>;//B(n,V<int>(n))

const ll INF=1LL<<60;
struct Edge{
  int to;
  ll w;
  Edge(int to,ll w):to(to),w(w){}
};
using Graph=VV<Edge>;
using pli=pair<ll,int>;
template<class T> bool chmin(T& a,T b){
  return ((a>b)?(a=b,true):(false));
}

int main(){
  ios::sync_with_stdio(false);
  std::cin.tie(nullptr);

  ll n,m,p,y;
  cin>>n>>m>>p>>y;
  
  Graph G(n);
  rep(i,m){
    int a,b,w;
    cin>>a>>b>>w;
    a--,b--;
    G[a].push_back(Edge(b,w));
    G[b].push_back(Edge(a,w));
  }

  V<ll> dist(n,INF);
  int s=0;
  dist[s]=0;
  priority_queue<pli,vector<pli>,greater<pli>> que;
  que.push({dist[s],s});
  while(!que.empty()){
    int v=que.top().second;
    ll d=que.top().first;
    que.pop();
    if(d>dist[v]) continue;
    for(auto e:G[v]){
      if(chmin(dist[e.to],dist[v]+e.w)){
        que.push({dist[e.to],e.to});
      }
    }
  }
  
  ll ans=0;
  rep(i,p){
    ll d,e;
    cin>>d>>e;
    d--;
    ll t=y-dist[d];
    if(t<=0) continue;
    ans=max(ans,t/e);
  }
  cout<<ans<<endl;

  return 0;
}