#include<bits/stdc++.h>

using namespace std;

using ll = long long;
using ld = long double;
//using ar2 = array<ll, 2>;

#define pb push_back
#define all(x) (x).begin(),(x).end()
#define rall(x) (x).rbegin(),(x).rend()
#define pop_cnt(s) ll(popcount(uint64_t(s)))
#define next_p(v) next_permutation(v.begin(), v.end())

template <typename T> inline bool chmin(T& a, const T& b) {bool c=a>b; if(a>b) a=b; return c;}
template <typename T> inline bool chmax(T& a, const T& b) {bool c=a<b; if(a<b) a=b; return c;}
template <typename T> inline T gcd(T a,T b) {return (b==0)?a:gcd(b,a%b);}
template <typename T> inline T lcm(T a, T b) {return (a*b)/gcd(a,b);}
template <typename T> using vc = vector<T>;
template <typename T> using vvc = vc<vc<T>>;
template <typename T> using vvvc = vc<vvc<T>>;
template <class T> using pq = priority_queue<T, vc<T>>;
template <class T> using pq_g = priority_queue<T, vc<T>, greater<T>>;

const ll INF = 2e18;
const vc<int> dx = {1, 1, 1, 0, 0, -1, -1, -1};
const vc<int> dy = {1, 0, -1, 1, -1, 1, 0, -1};

//ライブラリ貼るところ




//ここまで

void solve(){
    ll n, m, p, y; cin >> n >> m >> p >> y;
    vector<vector<pair<int, ll>>> graph(n);
    for(int i = 0; i < m; ++i){
        int a, b; cin >> a >> b;
        ll c; cin >> c;
        graph[--a].pb({--b, c});
        graph[b].pb({a, c});
    }
    vector<ll> cost(n, INF);
    cost[0] = 0;
    pq_g<pair<ll, int>> q;
    q.push({0, 0});
    while(!(q.empty())){
        auto [cs, t] = q.top();
        q.pop();
        if(cost[t] != cs) continue;
        for(auto [nt, ncs]: graph[t]){
            if(cost[nt] > cs + ncs){
                cost[nt] = cs + ncs;
                q.push({cs + ncs, nt});
            }
        }
    }
    ll ans = 0;
    for(int i = 0; i < p; ++i){
        int d; cin >> d;
        --d;
        ll e; cin >> e;
        ll z = y - cost[d];
        chmax(ans, z / e);
    }
    cout << ans << "\n";
}    











int main(){
    cin.tie( nullptr );
    ios::sync_with_stdio( false );
    cout << setprecision( 15 ) << fixed;

    int t = 1;
    for(int i = 0; i < t; ++i) solve();
}