import heapq


N, M, P, Y = map(int, input().split())
edges = [list(map(int, input().split())) for _ in range(M)]
shops = [list(map(int, input().split())) for _ in range(P)]

C = 10**18
Cost = [C]*N

graph = [[] for _ in range(N)]
for a, b, c in edges:
    graph[b-1].append((a-1, c))
    graph[a-1].append((b-1, c))

heap = [(0, 0)]
Cost[0] = 0

visited = [False]*N

while heap:
    c, x = heapq.heappop(heap)
    if visited[x]:
        continue
    visited[x] = True

    for u, c2 in graph[x]:
        if not visited[u] and c + c2 < Cost[u]:
            heapq.heappush(heap, [c+c2, u])
            Cost[u] = c+c2

answer = 0
for d, e in shops:
    remain = max(Y-Cost[d-1], 0)
    answer = max(remain//e, answer)

print(answer)