Random Number Generator (no repeat digits 1000-9999)

import random

a = [0,1,2,3,4,5,6,7,8,9]

b = random.randint(1,9)

string = ''

for index in range(4):
    string += str(a[b])
    del a[b]
    b = random.randint(0, len(a))

print string