<?php
[$N, $M, $P, $Y] = ints();
$di = new Di($N);
for($i = 0; $i < $M; $i++){
    [$f, $t, $c] = ints();
    $di->connect($f, $t, $c);
    $di->connect($t, $f, $c);
}
$di->solve(1);
$max = 0;
for($i = 1; $i <= $P; $i++){
    [$pos, $price] = ints();
    $money = max($Y - $di->distance[$pos],0);
    $cnt = intdiv($money, $price);
    chmax($max, $cnt);
}
echo $max;

class Di{
  public $pq;
  public $distance;
  public $G;
  public $from;
  public $inf = 10**18;
  function __construct($n) {
    $this->distance = array_fill(1, $n, $this->inf);
//    $this->from = array_fill(1,$n,0);
    $this->G = array_fill(1,$n,[]);
  }
  function connect($from, $to, $cost){
    if(isset($this->G[$from][$to])){
      $this->G[$from][$to] = min($this->G[$from][$to], $cost);
    }else{
      $this->G[$from][$to] = $cost;
    }
  }
  function solve($from){
    $pq = new SplPriorityQueue();
    $distance = $this->distance;
    $tfrom = $this->from;
    $G = $this->G;
    $pq->insert($from, 0);
    $distance[$from] = 0;
    while($pq->count()){
      $f = $pq->extract();
      if(!isset($map[$f])){
        $map[$f] = true;
        foreach($G[$f] as $t => $dist){
          $new = $distance[$f] + $dist;
          if($distance[$t] > $new){
            $distance[$t] = $new;
            //$tfrom[$t] = $f;
            $pq->insert($t, -$new);
          }
        }
      }
    }
    $this->distance = $distance;
    //$this->from = $tfrom;
  }
}

function str(){return trim(fgets(STDIN));}
function ints($n = false){
  if($n===false){
    $str = trim(fgets(STDIN));if($str == '')return [];
    return array_map('intval',explode(' ',$str));
  }else{$ret = [];for($i = 0; $i < $n; $i++)foreach(array_map('intval',explode(' ',trim(fgets(STDIN)))) as $j => $v)$ret[$j][] = $v;return $ret;}
}
function int(){return intval(trim(fgets(STDIN)));}
function chmax(&$a,$b){if($a<$b){$a=$b;return 1;}return 0;}
function chmin(&$a,$b){if($a>$b){$a=$b;return 1;}return 0;}
function isqrt($n):int{$res=intval(sqrt($n))+1;while($res*$res>$n)$res--;return $res;}
function popcount($x){$c=0;while($x){$x&=$x-1;++$c;}return$c;}
function swap(&$a,&$b){$tmp=$a;$a=$b;$b=$tmp;}
function o(...$val){
  if(count($val)==1)$val=array_shift($val);
  echo debug_backtrace()[0]['line'].")";
  if(is_array($val)){
    if(count($val) == 0)echo "empty array\n";
    elseif(!is_array(current($val)))echo "array: ",implode(" ", addIndex($val)),"\n";
    else{
      echo "array:array\n";
      if(isCleanArray($val))foreach($val as $row)echo implode(" ", addIndex($row)),"\n";
      else foreach($val as $i => $row)echo "[".$i."] ".implode(" ", addIndex($row)),"\n";
    }
  }else echo $val."\n";
}
function loadTree($N = false){
  if($N === false)$N = $GLOBALS['N'];
  return loadGraph($N, $N-1);
}
function loadGraph($N = false, $M = false, $both = true){
  if($N === false)$N = $GLOBALS['N'];
  if($M === false)$M = $GLOBALS['M'];
  $G = array_fill(1, $N, []);
  for($i = 0; $i < $M; $i++){
    $values = ints();
    if(count($values) == 2){
      list($a, $b) = $values;
      if($both == 0){
        $G[$a][] = $b;
      }elseif($both == 1){
        $G[$a][] = $b;
        $G[$b][] = $a;
      }else{
        $G[$b][] = $a;
      }
    }else{
      list($a, $b, $d) = $values;
      if($both == 0){
        $G[$a][] = [$b, $d];
      }elseif($both == 1){
        $G[$a][] = [$b, $d];
        $G[$b][] = [$a, $d];
      }else{
        $G[$b][] = [$a, $d];
      }
    }
  }
  return $G;
}