import sys
sys.setrecursionlimit(10**6)

N,M,P,Y = map(int,input().split())
G = [set() for _ in range(N)]
for _ in range(M):
    a,b,c = map(int,input().split())
    a,b = a-1,b-1
    G[a].add((b,c))
    G[b].add((a,c))

D = dict()
for _ in range(P):
    d,e = map(int,input().split())
    D[d] = e

def dfs(x,k,m):
    global Ans
    Ans = max(Ans,k//m)
    V[x] = True
    for g,c in G[x]:
        if not V[g]:
            e = 10**12
            if g in D:
                e = D[g]
            dfs(g,k-c,min(m,e))
    V[x] = False

Ans = 0
V = [False for _ in range(N)]
dfs(0,Y,10**18)
print(Ans)