N,M,P,Y=map(int,input().split())
edge = [[] for _ in range(N)]
for i in range(M):
    a,b,c=map(int,input().split())
    edge[a-1].append((b-1,c))
    edge[b-1].append((a-1,c))
l=[]
for i in range(P):
    l.append(list(map(int,input().split())))

import heapq
def dijkstra(s):
    hq=[]
    heapq.heappush(hq,[0,s])
    dist=[-1 for _ in range(N)]
    karidist=[10**20+1 for _ in range(N)]
    while len(hq)!=0:
        d,p=heapq.heappop(hq)
        if dist[p]==-1:
            dist[p]=d
            for e,c in edge[p]:
                if karidist[e]>d+c:
                    karidist[e]=d+c
                    heapq.heappush(hq,[d+c,e])
    return dist

dist = dijkstra(0)
ans=0
for d,e in l:
    ans=max(ans,(Y-dist[d-1])//e)
print(ans)