# library from https://prd-xxx.hateblo.jp/entry/2019/06/24/235844
import heapq
class Reverse:
    def __init__(self, val):
        self.val = val
        
    def __lt__(self, other):
        return self.val > other.val
        
    def __repr__(self):
        return repr(self.val)
    
class Heapq:
    def __init__(self, arr, desc = False):
        if desc:
            for i in range(len(arr)):
                arr[i] = Reverse(arr[i])
        self.desc = desc
        self.hq = arr
        heapq.heapify(self.hq)

    def pop(self):
        if self.desc: return heapq.heappop(self.hq).val
        else: return heapq.heappop(self.hq)

    def push(self, a):
        if self.desc: heapq.heappush(self.hq, Reverse(a))
        else: heapq.heappush(self.hq, a)

    def top(self):
        if self.desc: self.hq[0].val
        else: return self.hq[0]

n,m,p,y = map(int, input().split())
abc = [list(map(int, input().split())) for i in range(m)]
de = [list(map(int, input().split())) for i in range(p)]

graph = [[] for i in range(n)]

for a,b,c in abc:
    a -= 1
    b -= 1
    graph[a].append((b,c))
    graph[b].append((a,c))

# hachi = [1 << 60 for i in range(n)]
# for d,e in de:
#     d -= 1
#     hachi[d] = e

state = [1 << 60 for i in range(n)]
state[0] = 0

heap = Heapq([], desc=True)

heap.push((0, 0))


while heap.hq:
    cost, city = heap.pop()
    if state[city] < cost:
        continue

    for ncity, nc in graph[city]:
        ncost = cost + nc
        if state[ncity] <= ncost:
            continue
        state[ncity] = ncost
        heap.push((ncost, ncity))

ans = 0

for d,e in de:
    d -= 1
    cost = state[d]

    if cost > y:
        continue

    res = y - cost
    tans = res // e
    ans = max(ans, tans)

print(ans)