# input
import sys
input = sys.stdin.readline
II = lambda : int(input())
MI = lambda : map(int, input().split())
LI = lambda : list(map(int, input().split()))
SI = lambda : input().rstrip()
LLI = lambda n : [list(map(int, input().split())) for _ in range(n)]
LSI = lambda n : [input().rstrip() for _ in range(n)]
MI_1 = lambda : map(lambda x:int(x)-1, input().split())
LI_1 = lambda : list(map(lambda x:int(x)-1, input().split()))

def graph(n:int, m:int, dir:bool=False, index:int=-1) -> list[set[int]]:
    edge = [set() for i in range(n+1+index)]
    for _ in range(m):
        a,b = map(int, input().split())
        a += index
        b += index
        edge[a].add(b)
        if not dir:
            edge[b].add(a)
    return edge

def graph_w(n:int, m:int, dir:bool=False, index:int=-1) -> list[set[tuple]]:
    edge = [set() for i in range(n+1+index)]
    for _ in range(m):
        a,b,c = map(int, input().split())
        a += index
        b += index
        edge[a].add((b,c))
        if not dir:
            edge[b].add((a,c))
    return edge

mod = 998244353
inf = 1001001001001001001
ordalp = lambda s : ord(s)-65 if s.isupper() else ord(s)-97
ordallalp = lambda s : ord(s)-39 if s.isupper() else ord(s)-97
yes = lambda : print("Yes")
no = lambda : print("No")
yn = lambda flag : print("Yes" if flag else "No")
def acc(a:list[int]):
    sa = [0]*(len(a)+1)
    for i in range(len(a)):
        sa[i+1] = a[i] + sa[i]
    return sa

prinf = lambda ans : print(ans if ans < 1000001001001001001 else -1)
alplow = "abcdefghijklmnopqrstuvwxyz"
alpup = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
alpall = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
URDL = {'U':(-1,0), 'R':(0,1), 'D':(1,0), 'L':(0,-1)}
DIR_4 = [[-1,0],[0,1],[1,0],[0,-1]]
DIR_8 = [[-1,0],[-1,1],[0,1],[1,1],[1,0],[1,-1],[0,-1],[-1,-1]]
DIR_BISHOP = [[-1,1],[1,1],[1,-1],[-1,-1]]
prime60 = [2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59]
# import pypyjit
# pypyjit.set_param('max_unroll_recursion=-1')
# sys.setrecursionlimit(10**7)
# sys.set_int_max_str_digits(0)

n,m,p,y = MI()
edge = graph_w(n,m)

from heapq import heappop,heappush
inf = 1001001001001001001

def dijkstra(edge:list[set[int]], start:int = 0, goal:int = None):
    """
    O((node+edge)log(edge))
    """
    n = len(edge)
    dis = [inf]*n
    dis[start] = 0
    que = [(0, start)]
  
    while que:
        cur_dis, cur_node = heappop(que)

        if dis[cur_node] < cur_dis:
            continue

        for next_node, weight in edge[cur_node]:
            next_dis = cur_dis + weight

            if next_dis < dis[next_node]:
                dis[next_node] = next_dis
                heappush(que, (next_dis, next_node))
    
    if goal != None: return dis[goal]
    return dis

dis = dijkstra(edge,0)
ans = 0


for i in range(p):
    d,e = MI()
    d -= 1
    now = (y - dis[d])//e
    ans = max(ans,now)
    
print(ans)