#ifndef ONLINE_JUDGE
#define _GLIBCXX_DEBUG
#endif
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using namespace std;
#define rep(i, n) for (ll i = 0; i < (ll)(n); i++)
#define ll long long
#define bigmod 1000000007
#define P pair<ll,ll>
#define T tuple<ll,ll,ll>
#define nall(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()//逆順ソート
#define inf 2000000000000000000LL
#define mint modint998244353
#define mkp make_pair
#define mkt make_tuple
template<class X> using priority_minq = priority_queue<X, vector<X>, greater<X>>;//小さい順
ll min(ll a1,ll b1){if(a1>b1)return b1;else return a1;}
ll max(ll a1,ll b1){if(a1>b1)return a1;else return b1;}
#define chmax(x,y) x = max(x,y)
#define chmin(x,y) x = min(x,y)

//cout << fixed << setprecision(10); double型出力するときに使うやつ

ll n,m,p,y;
vector<P> g[200009];//to,cost
ll dat[200009];

int main(){
    cin >> n >> m >> p >> y;
    rep(i,m){
        ll a,b,c; cin >> a >> b >> c;
        g[a].push_back(mkp(b,c));
        g[b].push_back(mkp(a,c));
    }
    rep(i,n)dat[i+1] = 0;
    dat[1] = y;
    priority_queue<P> qu; qu.push(mkp(y,1));
    while(!qu.empty()){
        ll to,cost; tie(cost,to) = qu.top();
        qu.pop();
        if(cost < dat[to])continue;
        for(auto nex : g[to]){
            if(dat[nex.first] < cost - nex.second){
                dat[nex.first] = cost - nex.second;
                qu.push(mkp(dat[nex.first],nex.first));
            }
        }
    }
    ll ans = 0;
    rep(i,p){
        ll pos,cost; cin >> pos >> cost;
        chmax(ans,dat[pos]/cost);
    }
    cout << ans << endl;
}