from heapq import *
n,m,p,y=map(int,input().split())
#???i????????????????????max
connect=[[] for _ in range(n+1)]
for i in range(m):
    a,b,c=map(int,input().split())
    connect[a].append((b,c))
    connect[b].append((a,c))
store=[]
for i in range(p):
    a,b=map(int,input().split())
    store.append((a,b))
decide=[False]*(n+1)
que=[]
heappush(que,(-y,1))
inf=10**18
dist=[-inf]*(n+1)
dist[1]=y
while que:
    cost,now=heappop(que)
    cost*=-1
    if dist[now]!=cost:
        continue
    decide[now]=True
    for to,weight in connect[now]:
        if not decide[to]:
            if cost-weight>dist[to]:
                dist[to]=cost-weight
                heappush(que,(-(cost-weight),to))
ans=0
for i,j in store:
    if dist[i]>0:
        ans=max(ans,dist[i]//j)
print(ans)