solution.py

import json
import sys

board = json.loads(sys.stdin.read())

add_one = lambda x, y: x + y
subtract_one = lambda x, y: x - y
no_op = lambda x, y: x

transforms = (
    (no_op, add_one),              # horizontal
    (add_one, no_op),              # vertical
    (add_one, subtract_one),       # ul/lr
    (subtract_one, subtract_one)   # ll/ur
)

def transform(char, row, col, row_transform, col_transform):
    try:
        return all(
            board[row_transform(row, i)][col_transform(col, i)] == char
            for i in range(4)
        )
    except IndexError:
        return False

for row in range(len(board)):
    for col in range(len(board[0])):
        for char in ('X', 'O'):
            if any(transform(char, row, col, *t) for t in transforms):
                print 'Winner: %s' % char
                sys.exit(0)

print 'No winner.'