#include <bits/stdc++.h>
using namespace std;
using llg = long long;
const int maxNod = (int)(1e5) + 5;
const int maxEdges = 3 * (int)(1e5) + 5;
const int maxQueries = 3 * (int)(1e5) + 5;
const int maxTokens = 2 * maxQueries;
const llg BIG = (llg)(1e9) * (llg)(1e9);
int nbNod, nbEdges, nbCentrals, nbQueries;
llg rep[maxQueries];
int dsuRepr[maxNod];
int dsuSize[maxNod];
vector<int> tokenIn[maxNod];
int baseTok[maxTokens];
void dsuInit() {
  for (int nod = 0; nod < maxNod; ++nod) {
    dsuRepr[nod] = nod;
    dsuSize[nod] = 1;
  }
}
int dsuFind(int x) {
  if (x != dsuRepr[x]) {
    dsuRepr[x] = dsuFind(dsuRepr[x]);
  }
  return dsuRepr[x];
}
void dsuMerge(int small, int big, llg curCap) {
  small = dsuFind(small);
  big = dsuFind(big);
  if (small == big) return;
  if (dsuSize[small] > dsuSize[big]) swap(small, big);
  for (int token : tokenIn[small]) {
    int oth = token ^ 1;
    int query = token / 2;
    if (dsuFind(baseTok[oth]) == big) {
      rep[query] = curCap;
    }
    tokenIn[big].push_back(token);
  }
  tokenIn[small].clear();
  tokenIn[small].shrink_to_fit();
  dsuSize[big] += dsuSize[small];
  dsuRepr[small] = big;
}
vector<pair<int, llg>> adj[maxNod];
llg mdis[maxNod];
void dijkstra() {
  priority_queue<pair<llg, int>> pq;
  for (int nod = 0; nod < nbNod; ++nod) {
    if (nod < nbCentrals) {
      pq.push({0, nod});
    } else {
      mdis[nod] = BIG;
    }
  }
  while (!pq.empty()) {
    llg dist = -pq.top().first;
    int nod = pq.top().second;
    pq.pop();
    if (dist != mdis[nod]) continue;
    for (auto rawNeigh : adj[nod]) {
      int neighbor = rawNeigh.first;
      llg newDis = dist + rawNeigh.second;
      if (newDis < mdis[neighbor]) {
        mdis[neighbor] = newDis;
        pq.push({-newDis, neighbor});
      }
    }
  }
}
struct Edge {
  llg weight, n1, n2;
};
bool operator<(Edge a, Edge b) { return a.weight < b.weight; }
vector<Edge> tabEdges;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  dsuInit();
  cin >> nbNod >> nbEdges >> nbCentrals >> nbQueries;
  for (int iEdge = 0; iEdge < nbEdges; ++iEdge) {
    int n1, n2;
    llg weight;
    cin >> n1 >> n2 >> weight;
    --n1;
    --n2;
    adj[n1].emplace_back(n2, weight);
    adj[n2].emplace_back(n1, weight);
    tabEdges.push_back({weight, n1, n2});
  }
  for (int query = 0; query < nbQueries; ++query) {
    int n1, n2;
    cin >> n1 >> n2;
    --n1;
    --n2;
    int tk1 = 2 * query, tk2 = 2 * query + 1;
    baseTok[tk1] = n1;
    baseTok[tk2] = n2;
    tokenIn[n1].push_back(tk1);
    tokenIn[n2].push_back(tk2);
  }
  dijkstra();
  for (auto &edge : tabEdges) {
    edge.weight += mdis[edge.n1] + mdis[edge.n2];
  }
  sort(tabEdges.begin(), tabEdges.end());
  for (auto edge : tabEdges) {
    dsuMerge(edge.n1, edge.n2, edge.weight);
  }
  for (int query = 0; query < nbQueries; ++query) {
    cout << rep[query] << "\n";
  }
}
