#include <bits/stdc++.h>
using namespace std;

// template {{{
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define mt make_tuple
#define lb lower_bound
#define ub upper_bound
#define f first
#define s second
#define resz resize

#define sz(x) int((x).size())
#define all(x) (x).begin(), (x).end()

#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define F0R(i, a) for (int i = 0; i < (a); i++)
#define FORd(i, a, b) for (int i = (b)-1; i >= (a); i--)
#define F0Rd(i, a) for (int i = (a)-1; i >= 0; i--)
#define trav(a, x) for (auto& a : x)

#define sort_by(x, y) sort(all(x), [&](const auto& a, const auto& b) { return y; })

using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using vb = vector<bool>;
using vd = vector<double>;
using vs = vector<string>;

using pii = pair<int, int>;
using pll = pair<ll, ll>;
using pdd = pair<double, double>;

using vpii = vector<pii>;
using vvpii = vector<vpii>;
using vpll = vector<pll>;
using vvpll = vector<vpll>;
using vpdd = vector<pdd>;
using vvpdd = vector<vpdd>;

template<typename T> void ckmin(T& a, const T& b) { a = min(a, b); }
template<typename T> void ckmax(T& a, const T& b) { a = max(a, b); }

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());

namespace __input {
    template<class T1, class T2> void re(pair<T1,T2>& p);
    template<class T> void re(vector<T>& a);
    template<class T, size_t SZ> void re(array<T,SZ>& a);

    template<class T> void re(T& x) { cin >> x; }
    void re(double& x) { string t; re(t); x = stod(t); }
    template<class Arg, class... Args> void re(Arg& first, Args&... rest) {
        re(first); re(rest...);
    }

    template<class T1, class T2> void re(pair<T1,T2>& p) { re(p.f,p.s); }
    template<class T> void re(vector<T>& a) { F0R(i,sz(a)) re(a[i]); }
    template<class T, size_t SZ> void re(array<T,SZ>& a) { F0R(i,SZ) re(a[i]); }
}
using namespace __input;

namespace __output {
    template<class T1, class T2> void pr(const pair<T1,T2>& x);
    template<class T, size_t SZ> void pr(const array<T,SZ>& x);
    template<class T> void pr(const vector<T>& x);
    template<class T> void pr(const deque<T>& x);
    template<class T> void pr(const set<T>& x);
    template<class T1, class T2> void pr(const map<T1,T2>& x);

    template<class T> void pr(const T& x) { cout << x; }
    template<class Arg, class... Args> void pr(const Arg& first, const Args&... rest) {
        pr(first); pr(rest...);
    }

    template<class T1, class T2> void pr(const pair<T1,T2>& x) {
        pr("{",x.f,", ",x.s,"}");
    }
    template<class T, bool pretty = true> void prContain(const T& x) {
        if (pretty) pr("{");
        bool fst = 1; for (const auto& a: x) pr(!fst?pretty?", ":" ":"",a), fst = 0;
        if (pretty) pr("}");
    }
    template<class T> void pc(const T& x) { prContain<T, false>(x); pr("\n"); }
    template<class T, size_t SZ> void pr(const array<T,SZ>& x) { prContain(x); }
    template<class T> void pr(const vector<T>& x) { prContain(x); }
    template<class T> void pr(const deque<T>& x) { prContain(x); }
    template<class T> void pr(const set<T>& x) { prContain(x); }
    template<class T1, class T2> void pr(const map<T1,T2>& x) { prContain(x); }

    void ps() { pr("\n"); }
    template<class Arg> void ps(const Arg& first) {
        pr(first); ps();
    }
    template<class Arg, class... Args> void ps(const Arg& first, const Args&... rest) {
        pr(first," "); ps(rest...);
    }
}
using namespace __output;

#define TRACE(x) x
#define __pn(x) pr(#x, " = ")
#define pd(...) __pn((__VA_ARGS__)), ps(__VA_ARGS__), cout << flush

namespace __algorithm {
    template<typename T> void dedup(vector<T>& v) {
        sort(all(v)); v.erase(unique(all(v)), v.end());
    }
    template<typename T> typename vector<T>::iterator find(vector<T>& v, const T& x) {
        auto it = lower_bound(all(v), x); return it != v.end() && *it == x ? it : v.end();
    }
    template<typename T> size_t index(vector<T>& v, const T& x) {
        auto it = find(v, x); assert(it != v.end() && *it == x); return it - v.begin();
    }
    template<typename C, typename T, typename OP> vector<T> prefixes(const C& v, T id, OP op) {
        vector<T> r(sz(v)+1, id); F0R (i, sz(v)) r[i+1] = op(r[i], v[i]); return r;
    }
    template<typename C, typename T, typename OP> vector<T> suffixes(const C& v, T id, OP op) {
        vector<T> r(sz(v)+1, id); F0Rd (i, sz(v)) r[i] = op(v[i], r[i+1]); return r;
    }
}
using namespace __algorithm;

struct monostate {
    friend istream& operator>>(istream& is, const __attribute__((unused))monostate& ms) { return is; }
    friend ostream& operator<<(ostream& os, const __attribute__((unused))monostate& ms) { return os; }
} ms;

template<typename W=monostate> struct wedge {
    int u, v, i; W w;
    wedge<W>(int _u=-1, int _v=-1, int _i=-1) : u(_u), v(_v), i(_i) {}
    int operator[](int loc) const { return u ^ v ^ loc; }
    friend void re(wedge& e) { re(e.u, e.v, e.w); --e.u, --e.v; }
    friend void pr(const wedge& e) { pr(e.u, "<-", e.w, "->", e.v); }
};

namespace __io {
    void setIn(string s) { freopen(s.c_str(),"r",stdin); }
    void setOut(string s) { freopen(s.c_str(),"w",stdout); }
    void setIO(string s = "") {
        ios_base::sync_with_stdio(0); cin.tie(0);
        cout << fixed << setprecision(15);
        if (sz(s)) { setIn(s+".in"), setOut(s+".out"); }
    }
}
using namespace __io;
// }}}

// union_find {{{
template<typename V = monostate, typename I = function<V(int)>>
struct union_find {
    struct node {
        int par, rnk, size; vi elts; V state;
        node(int id = 0) : par(id), rnk(0), size(1), elts({id}) {}
        template<typename F> void merge(node& o, F proc) {
            proc(par, o.par);
            if (rnk == o.rnk) rnk++;
            size += o.size;
            if (sz(elts) < sz(o.elts)) swap(elts, o.elts);
            elts.insert(elts.end(), all(o.elts));
        }
    };

    vector<node> uf;
    union_find(int N = 0, I init = [](int i) { return V{}; }) : uf(N) {
        for (int i = 0; i < N; i++) {
            uf[i] = node(i);
            uf[i].state = init(i);
        }
    }

    int rep(int i) {
        if (i != uf[i].par)
            uf[i].par = rep(uf[i].par);
        return uf[i].par;
    }

    template<typename F = function<void(int, int)>>
    bool unio(int a, int b, F proc = [](int a, int b) {}) {
        a = rep(a), b = rep(b);
        if (a == b) return false;
        if (uf[a].rnk < uf[b].rnk) swap(a, b);
        uf[a].merge(uf[b], proc);
        uf[b].par = a;
        return true;
    }

    node& operator[](int i) { return uf[rep(i)]; }

    friend void pr(const union_find<V, I>& u) {
        pr("{"); bool f = 1;
        for (int i = 0; i < sz(u.uf); i++) if (u.uf[i].par == i) {
            if (!f) pr(", "); else f = 0;
            pr("[ ", i, " | rank=", u.uf[i].rnk, " size=", u.uf[i].size,
                    " elts=", u.uf[i].elts, " ", u.uf[i].state, "]");
        }
        pr("}");
    }
};
// }}}

int main() {
    setIO();

    int N, M, K, Q; re(N, M, K, Q);

    struct E { int u, v, w; };
    vector<E> edges(M);
    vvi adj(N);
    F0R (i, M) {
        E& e = edges[i];
        re(e.u, e.v, e.w); --e.u, --e.v;
        adj[e.u].pb(i);
        adj[e.v].pb(i);
    }

    vi nearest(N, INT_MAX);
    priority_queue<pii, vpii, greater<pii>> pq;
    F0R (i, K) nearest[i] = 0, pq.push({ 0, i});
    while (!pq.empty()) {
        auto [dist, loc] = pq.top(); pq.pop();
        for (int ei : adj[loc]) {
            E& e = edges[ei];
            int nbr = e.u ^ e.v ^ loc;
            if (nearest[loc] + e.w < nearest[nbr]) {
                nearest[nbr] = nearest[loc] + e.w;
                pq.push({ nearest[nbr], nbr });
            }
        }
    }

    vi ans(Q);
    vvpii queries(N);
    F0R (q, Q) {
        int a, b; re(a, b); --a, --b;
        queries[a].eb(b, q);
        queries[b].eb(a, q);
    }

    union_find<> uf(N);

    sort_by(edges, nearest[a.u] + nearest[a.v] + a.w <
                   nearest[b.u] + nearest[b.v] + b.w);

    for (auto [u, v, w] : edges) {
        uf.unio(u, v, [&](int a, int b) {
            ll cost = ll(w) + nearest[u] + nearest[v];

            vpii &qa = queries[a], &qb = queries[b];
            int id = a;
            if (sz(qa) < sz(qb)) { swap(qa, qb); id = b; }

            trav (q, qb) { if (uf.rep(q.f) == id) ans[q.s] = cost; }
            qa.insert(qa.end(), all(qb));
            qb.clear();
        });
    }

    trav (e, ans) ps(e);

    // did you check N=1? did you mix up N,M?
    return 0;
}

