import heapq
import queue

N , M , Q , Y = map( int , input().split() )

R = [ [] for _ in range(N) ]
for _ in range(M) :
    a , b , c = map( int , input().split() )
    a -= 1
    b -= 1
    R[a].append([b,c])
    R[b].append([a,c])

INF = 10**20
P = [ INF ] * N
for _ in range(Q) :
    d , e = map( int , input().split() )
    P[d-1] = e

check = [ True ] * N
num = [ 0 ] * N
dis = [ -1 ] * N

num[0] = Y // P[0]
dis[0] = 0

s = []
heapq.heappush( s , [ 0 , 0 ] )

while len(s) != 0 :
    d , p = heapq.heappop( s )
    if check[p] :
        dis[p] = d
        num[p] = ( Y - d ) // P[p]
        check[p] = False
        for q , c  in R[p] :
            if check[q] :
                heapq.heappush( s , [ d + c , q ] )
                
print( max(num) )