Steven, I ran this:
import time, collections, itertools
t = time.clock()
q,w,e,r,sch = [],[],[],[],0
h = collections.defaultdict(itertools.repeat(0).next)
f = open("D:/m4000.txt","rt")
for o in range(int(f.readline())):
row = map(int, f.readline().split())
q.append(row[0])
w.append(row[1])
e.append(row[2])
r.append(row[3])
f.close()
for x in q:
for y in w:
h[x+y] += 1
for x in e:
for y in r:
sch += h[-(x + y)]
q,w,e,r,h = None,None,None,None,None
print sch
print time.clock() - t
========= and it almost froze my PC...
=== but it was faster than my code on input file with 1000 rows:
====== 2.00864607094s VS 3.14631077413s
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