from heapq import heapify, heappop, heappush
N, M, P, Y = map(int, input().split())

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

DE = []
for i in range(P):
    d, e = map(int, input().split())
    d -= 1
    DE.append((d, e))

def dijk(G, start=0):
    N = len(G)
    dist = [10**18]*N
    visited = [False]*N
    que = []
    dist[start] = 0
    heappush(que, (0, start))
    while que:
        _, now = heappop(que)
        if visited[now] == True:
            continue
        visited[now] = True
        for to, weight in G[now]:
            if dist[now] + weight < dist[to]:
                dist[to] = dist[now] + weight
                heappush(que, (dist[now]+weight, to))
    return dist

stt = 0
dist = dijk(G, stt)
# print(dist)
ans = 0
for i in range(P):
    d, e = DE[i]
    x = (Y - dist[d]) // e
    ans = max(ans, x)
print(ans)