#include <bits/stdc++.h>
using namespace std;
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
//#pragma GCC optimize("Ofast,unroll-loops,fast-math,O3")
#define pb push_back
#define ll long long
#define f first
#define s second
#define sz(v) int(v.size())
#define all(v) v.begin(),v.end()
int mod = 1e9 + 7;
const int N = 50000 + 100,N1 = 5000;
const int inf = 1e18;
vector <pair<int,int>> g[N];
ll d[N];
void solve(){
int k,n,m,q,s,f;
cin >> k >> n >> m >> q;
for(int i = 0; i <= n; i++)d[i] = inf;
for(int i = 1; i <= m; i++){
int u,v,w;
cin >>u >> v >> w;
g[u].pb({v,w});
}
set <pair <ll,int>> st;
set <int> st1;
while(q--){
cin >> s >> f;
st.insert({0,s});
d[s] = 0;
while(!st.empty()){
auto u = (*st.begin()).s;
st1.insert(u);
st.erase(st.begin());
for(auto [to,w] : g[u]){
if(d[to] > d[u] + w){
st.erase({d[to],to});
d[to] = d[u] + w;
st.insert({d[to],to});
}
}
}
cout << (d[f] == inf ? -1 : d[f]) << '\n';
while(!st1.empty()){
d[*st1.begin()] = inf;
st1.erase(st1.begin());
}
}
}
signed main(){
//freopen("time.in", "r", stdin);
//freopen("time.out", "w", stdout);
ios_base::sync_with_stdio(0);
cin.tie(0);cout.tie(0);
int t1 = 1;
while(t1--){
solve();
}
}
