Troubles with program

[Harambe]

Yes, in ascending order. Meanwhile I was thinking about this:

import random
number=random.randrange(1500)+1
letter=random.choice(['a','b','c','d','e','f','g','h'])
dot='.'
for i in range(100):
    number=random.randrange(1500)+1
    letter1=random.choice(['a','b','c','d','e','f','g','h'])
    letter2=random.choice(['a','b','c','d','e','f','g','h'])
    letter3=random.choice(['a','b','c','d','e','f','g','h'])
    letter4=random.choice(['a','b','c','d','e','f','g','h'])
    if letter1='a':
        print(letter2=random.choice(['b','c','d','e','f','g','h'])
    elif letter1='b':
        print(letter2=random.choice(['a','c','d','e','f','g','h'])
    else letter1='c':
        print(letter2=random.choice(['a','b','d','e','f','g','h'])

I want to determinate all 4 letters like this to avoid repeating. But I cant find why the program is not working now.