import sys
input = lambda: sys.stdin.readline().rstrip()

import heapq

def main():
    # 入力
    N, M, P, Y = map(int, input().split())
    edges = [[] for _ in range(N)]
    for _ in range(M):
        A, B, C = map(int, input().split())
        A, B = A - 1, B - 1
        edges[A].append((B, C))
        edges[B].append((A, C))
    shop = [10**18+1]*N
    for _ in range(P):
        D, E = map(int, input().split())
        shop[D-1] = E
    # 計算・出力
    dist = [10**18]*N
    dist[0] = 0
    hq = [(0, 0)]
    while hq:
        prevdist, prevnnode = heapq.heappop(hq)
        if dist[prevnnode] != prevdist: continue
        for nextnode, ddist in edges[prevnnode]:
            nextdist = prevdist + ddist
            if nextdist < dist[nextnode]:
                dist[nextnode] = nextdist
                heapq.heappush(hq, (nextdist, nextnode))
    ans = 0
    for i in range(N):
        nowY = Y - dist[i]
        res = nowY // shop[i]
        ans = max(ans, res)
    print(ans)

if __name__ == "__main__":
    main()