// InterestingLSY
// 2659723130
// 《幽灵公主》是真的好看！
#ifdef intLSY
#include <intlsy/stdc++.h>
#else
#include <bits/stdc++.h>
#endif
#include <bits/c++0x_warning.h>
using namespace std;
#define il inline
#define elif else if

// Type
#define ld double
#define ll long long
#define ull unsigned ll

// Vector
#define vc vector
#define Pb push_back
#define Pf push_front
#define Eb emplace_back
#define All(x) x.begin(),x.end()
#define AllRev(x) x.rbegin(),x.rend()

// Memory
#define Ms(_data) memset(_data,0,sizeof(_data))
#define Msn(_data,_num) memset(_data,_num,sizeof(_data))

// Template
#define _cl class
#define _tp template
#define _tyn typename

// Pair
#define Mp make_pair
#define F first
#define S second
#define pii pair<int,int>
#define pli pair<ll,int>
#define pil pair<int,ll>
#define pll pair<ll,ll>

// Loop
#define For(i,j) for( int (i) = 1 ; (i) <= (j) ; ++(i) )
#define For0(i,j) for( int (i) = 0 ; (i) < (j) ; ++(i) )
#define Forx(i,j,k) for( int (i) = (j) ; (i) <= (k) ; ++(i) )
#define Forstep(i,j,k,st) for( int (i) = (j) ; (i) <= (k) ; (i) += (st) )
#define fOR(i,j) for( int (i) = (j) ; (i) >= 1 ; --(i) )
#define fOR0(i,j) for( int (i) = (j)-1 ; (i) >= 0 ; --(i) )
#define fORx(i,j,k) for( int (i) = (k) ; (i) >= (j) ; --(i) )

// Read
struct InputReader{
	#define bs 1048576
	char buf[bs]; int p;
	il InputReader(){ p = bs; }
	il void Flush(){ p = 0; fread(buf,1,bs,stdin); }
	il char C(){ if(p >= bs) Flush(); return buf[p++]; }
	il char Readnum(){ char ch = C(); while( !isdigit(ch) && ch != '-' ) ch = C(); return ch; }
	il void Readalpha( char &c ){ c = C(); while( !isalpha(c) ) c = C(); }
	int operator() (){
		int ans = 0, fu = 1; char ch = Readnum();
		if( ch == '-' ) fu = -1, ch = C();
		while( ch >= '0' && ch <= '9' ){
			ans = ans*10 + ch-'0';
			ch = C();
		}
		return ans * fu;
	}
	ll Readll(){
		ll ans = 0LL, fu = 1LL; char ch = Readnum();
		if( ch == '-' ) fu = -1LL, ch = C();
		while( ch >= '0' && ch <= '9' ){
			ans = ans*10LL + ch-'0';
			ch = C();
		}
		return ans * fu;
	}
	il void Readstring( string &x ){
		x.clear(); char ch = C();
		while( !isdigit(ch)  &&  !isalpha(ch)  &&  ch != '#'  &&  ch != '.' ) ch = C();
		while( isdigit(ch)  ||  isalpha(ch)  ||  ch == '#'  ||  ch == '.' ){
			x += ch;
			ch = C();
		}
	}
	il void Readchstring( char s[] ){
		int len = 0; char ch = C();
		while( !isdigit(ch)  &&  !isalpha(ch)  /*&&  ch != '*'  &&  ch != '?'*/ ) ch = C();
		while( isdigit(ch)  ||  isalpha(ch)  /*||  ch == '*'  ||  ch == '?'*/ ){
			s[len++] = ch;
			ch = C();
		}
		s[len] = '\0';
	}
	il void Specialread( char &c ){
		c = C();
		while( !isdigit(c) && !isalpha(c) && c != '#'  &&  c != '.'  &&  c != '='  &&  c != 'B' ) c = C();
	}
	#undef bs
}In;
il void Read( int &x ){ x = In(); }
il void Read( int &x, int &y ){ x = In(); y = In(); }
il void Read( int &x1 , int &x2 , int &x3 ){ x1 = In(); x2 = In(); x3 = In(); }
il void Read( int &x1 , int &x2 , int &x3 , int &x4 ){ x1 = In(); x2 = In(); x3 = In(); x4 = In(); }
il void Read( ll &x ){ x = In.Readll(); }
il void Read( ll &x, ll &y ){ x = In.Readll(); y = In.Readll(); }
il void Read( ll &x1 , ll &x2 , ll &x3 ){ x1 = In.Readll(); x2 = In.Readll(); x3 = In.Readll(); }
il void Read( ll &x1 , ll &x2 , ll &x3 , ll &x4 ){ x1 = In.Readll(); x2 = In.Readll(); x3 = In.Readll(); x4 = In.Readll(); }
_tp<_tyn T> void Read( T a[] , int st , int ed ){ Forx(i,st,ed) Read(a[i]); }
#define iRead(k) int k; Read(k);
#define iRead2(a,b) iRead(a); iRead(b);
#define iRead3(a,b,c) iRead2(a,b); iRead(c);
#define iRead4(a,b,c,d) iRead2(a,b); iRead2(c,d);
#define lRead(k) ll k; Read(k);
#define lRead2(a,b) lRead(a); lRead(b);
#define lRead3(a,b,c) lRead2(a,b); lRead(c);
#define lRead4(a,b,c,d) lRead2(a,b); lRead2(c,d);

// File
#define Fin(a) freopen(a,"r",stdin)
#define Fout(a) freopen(a,"w",stdout)
il void FILEIO(){
	#ifdef intLSY
		Fin("in.in");
	#endif
}
il void FILEIO( string pname ){
	#ifndef intLSY
		Fin((pname+".in").c_str());
		Fout((pname+".out").c_str());
	#else
		Fin("in.in");
	#endif
}
il void FILEIO_OICONTEST( string pname ){
	Fin((pname+".in").c_str());
	#ifndef intLSY
		Fout((pname+".out").c_str());
	#endif
}
void Printtime(){
	#ifdef intLSY
		double _timeuse = clock()* 1000.0 / CLOCKS_PER_SEC;
		fprintf(stderr,"\n\nTime usage:\n%.0lf ms\n",_timeuse);
	#endif
}
void END(){ Printtime(); exit(0); }
_tp<_tyn T>void END( T mes ){ cout << mes << endl; END(); }

// Debug
#define B cerr << "BreakPoint" << endl;
#define O(x) cerr << #x << " " << (x) << endl;
#define o(x) cerr << #x << " " << (x) << "  ";
#if __cplusplus >= 201703L
template<typename T, typename... Ts>
void P( T t0 , Ts... ts ){
    cerr << t0 << " ";
    if constexpr(sizeof...(ts)>0) P(ts...);
    else cerr << endl;
}
#else
template<typename T, typename... Ts>
void P( T t0 , Ts... ts ){
    // I love cpp17.
    // Fuck you.
}
#endif
#define Msz(x) cerr << "Sizeof " << #x << " " << sizeof(x)/1024/1024 << " MB" << endl;
_tp<_tyn T>void Print( T a[] , int s , int t , char sp = ' ' , char ed = '\n' ){
	if( s > t ) return;
	for( int i = s ; i < t ; i++ )
		cout << a[i] << sp;
	cout << a[t] << ed;
	cout.flush();
}
_tp<_tyn T>void Print( T a , int s = 0 , int t = -1 , char sp = ' ' , char ed = '\n' ){
	if( t == -1 ) t = a.size()-1;
	for( int i = s ; i <= t ; i++ )
		cout << a[i] << sp;
	cout << ed;
	cout.flush();
}

// Optimize
#define Max(a,b) ((a)>(b)?(a):(b))
#define Min(a,b) ((a)<(b)?(a):(b))
#define Mymax(a,b) (a) = max((a),(b))
#define Mymin(a,b) (a) = min((a),(b))
#define MyMax(a,b) (a) = Max((a),(b))
#define MyMin(a,b) (a) = Min((a),(b))
#define INF (0x3f3f3f3f)
#define LINF ((long long)(0x3f3f3f3f3f3f3f3f))
#define Sqr(x) ((x)*(x))
#define Lowbit(a) ((a)&(-(a)))
vc<int> Range( int l , int r ){
	vc<int> ret{};
	Forx(i,l,r) ret.Pb(i);
	return ret;
}
mt19937 Rand(0xe38195e38293);	// さん
#include <functional>
///////////////////////////////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////////////////////////////
constexpr int MAXN = 131072;
constexpr int LOG = 18;
struct Edge{ int to; ll cost; };
struct E{ int f,t; ll c; };

int n,m,k,q;
vc<Edge> e[MAXN];
vc<E> edges;

ll d[MAXN];
void Dijkstra(){
	priority_queue<pii,vc<pii>,greater<pii>> pq;
	bool vis[MAXN];

	Msn(d,INF);
	For(i,k){
		d[i] = 0;
		pq.push({0,i});
	}

	while(!pq.empty()){
		int u = pq.top().S;
		pq.pop();
		if(vis[u]) continue;
		vis[u] = 1;
		for( auto [v,cost] : e[u] ){
			if( d[v] > d[u] + cost ){
				d[v] = d[u] + cost;
				pq.push({d[v],v});
			}
		}
	}
}

vc<Edge> ne[MAXN];
void Kruskal(){
	int dsu[MAXN];
	For(i,n) dsu[i] = i;
	function<int(int)> Findfa = [&]( int pos ){
		return dsu[pos] == pos ? pos : dsu[pos] = Findfa(dsu[pos]);
	};
	function<bool(int,int)> Unite = [&]( int a , int b ){
		a = Findfa(a); b = Findfa(b);
		if( a == b ) return 0;
		dsu[a] = b;
		return 1;
	};

	for( auto &e : edges ) e.c = d[e.f] + d[e.t] + e.c;
	sort(All(edges),[]( E a , E b ){ return a.c < b.c; });

	for( auto e : edges ){
		if(Unite(e.f,e.t)){
			ne[e.f].Pb({e.t,e.c});
			ne[e.t].Pb({e.f,e.c});
			// P(e.f,e.t,e.c);
		}
	}
}

int up[MAXN][LOG];
ll upmx[MAXN][LOG];
int dep[MAXN];
void PreDfs( int pos , int fa , ll faw ){
	dep[pos] = dep[fa] + 1;
	up[pos][0] = fa;
	upmx[pos][0] = faw;
	For(i,LOG-1){
		up[pos][i] = up[up[pos][i-1]][i-1];
		upmx[pos][i] = max( upmx[pos][i-1] , upmx[up[pos][i-1]][i-1] );
	}
	// O(pos);
	// Print(upmx[pos],0,3);
	for( auto [v,cost] : ne[pos] ){
		if( v == fa ) continue;
		PreDfs(v,pos,cost);
	}
}

pil Lca( int a , int b ){
	if( dep[a] < dep[b] ) swap(a,b);
	ll ans = 0;
	fOR0(i,LOG){
		int v = up[a][i];
		if( dep[v] >= dep[b] ){
			Mymax(ans,upmx[a][i]);
			a = v;
		}
	}
	// O(ans);
	if( a == b ) return {a,ans};
	fOR0(i,LOG){
		int u = up[a][i], v = up[b][i];
		if( u != v ){
			Mymax(ans,upmx[a][i]);
			Mymax(ans,upmx[b][i]);
			a = u; b = v;
		}
	}
	Mymax(ans,upmx[a][0]);
	Mymax(ans,upmx[b][0]);
	a = up[a][0];
	b = up[b][0];
	return {a,ans};
}

int main(){
	FILEIO();
	ios_base::sync_with_stdio(false);
	cout.tie(0);

	Read(n,m,k,q);
	For(i,m){
		iRead3(a,b,c);
		e[a].Pb({b,c});
		e[b].Pb({a,c});
		edges.Pb({a,b,c});
	}

	Dijkstra();
	// Print(d,1,n);

	Kruskal();

	PreDfs(1,0,0);

	For(qwq,q){
		iRead2(a,b);
		auto [lca,ans] = Lca(a,b);
		cout << ans << endl;
	}

	END();
}
