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

const int INF = 0x3f3f3f3f;
const int N = 1e5 + 10;

int n, m, k, q;
int dis[N];

bool vis[N];

int cnt;

struct node {
	int u, v, c; 
} edge[N * 6];

vector < pair <int, int> > g[N];
vector < pair <int, int> > e[N];

void dijkstra () {
	for (int i = 1; i <= n; i++) dis[i] = INF;
	priority_queue < pair <int, int> > q;
	for (int i = 1; i <= k; i++) dis[i] = 0, q.push (make_pair (0, i));
	while (!q.empty()) {
		int u = q.top().second;
		q.pop();
		if (vis[u]) continue;
		vis[u] = 1;
		for (int i = 0; i < g[u].size(); i++) {
			int v = g[u][i].first;
			int c = g[u][i].second;
			if (dis[v] > dis[u] + c) {
				dis[v] = dis[u] + c;
				q.push (make_pair (-dis[v], v));
			}
		}
	}
	for (int u = 1; u <= n; u++)
		for (int i = 0; i < g[u].size(); i++)
			edge[++cnt] = (node) {u, g[u][i].first, g[u][i].second + dis[u] + dis[g[u][i].first]};
}

int fa[N];

int find (int x) {
	return (fa[x] == x ? x : fa[x] = find (fa[x]));
}

bool cmp (node a, node b) {
	return a.c < b.c;
}

void kruskal () {
	int len = 1;
	sort (edge + 1, edge + 1 + cnt, cmp);
	for (int i = 1; i <= n; i++) fa[i] = i; 
	for (int i = 1; i <= cnt; i++) {
		int c = edge[i].c;
		int x = edge[i].u;
		int y = edge[i].v;
		int fx = find (x);
		int fy = find (y);
		if (fx == fy) continue;
		fa[fx] = fy;
		len++;
		e[x].push_back (make_pair (y, c));
		e[y].push_back (make_pair (x, c));
		if (len == n) return;
	}
}

int dep[N];
int maxn[N][21]; 
int f[N][21];

void dfs (int u, int father) {
	f[u][0] = father;
	dep[u] = dep[father] + 1;
	for (int i = 0; i < e[u].size(); i++) {
		int v = e[u][i].first;
		int c = e[u][i].second;
		if (v == father) continue;
		maxn[v][0] = c;
		dfs (v, u);
	}
	for (int i = 1; i <= 20; i++)
		f[u][i] = f[f[u][i - 1]][i - 1], maxn[u][i] = max (maxn[u][i - 1], maxn[f[u][i - 1]][i - 1]);
}

void slove () {
	dijkstra ();
	kruskal ();
	dfs (1, 0);
}

int LCA (int x, int y) {
	int ans = 0;
	if (dep[x] < dep[y]) swap (x, y);
	for (int i = 20; i >= 0; i--)
		if (dep[f[x][i]] > dep[y]) ans = max (ans, maxn[x][i]), x = f[x][i];
	ans = max (ans, maxn[x][0]);
	x = f[x][0];
	if (x == y) return ans;
	for (int i = 20; i >= 0; i--)
		if (f[x][i] != f[y][i]) ans = max (ans, max (maxn[x][i], maxn[y][i])), x = f[x][i], y = f[y][i];
	ans = max (ans, max (maxn[x][0], maxn[y][0]));
	return ans;
}

signed main () {
//	freopen ("robot.in", "r", stdin);
//	freopen ("robot.out", "w", stdout);
	ios :: sync_with_stdio (false);
	cin.tie (0);
	cout.tie (0);
	cin >> n >> m >> k >> q;
	for (int i = 1; i <= m; i++) {
		int u, v, c;
		cin >> u >> v >> c;
		g[u].push_back (make_pair (v, c));
		g[v].push_back (make_pair (u, c));
	}
	slove ();
	while (q--) {
		int x, y;
		cin >> x >> y;
		cout << LCA (x, y) << endl;
	}
	return 0;
}