This Python code will generate the required 4-character (Unique_code) serial numbers:

#!/usr/bin/python3
import re

for i in range(1,10000):
    if (i < 1000):
       print ("i =", str(i).zfill(4))
    else:
       m = re.findall(r'(\d)(\d\d\d)', str(i))
       code = 64+int(m[0][0])
       print ("i =",i, "Unique_code =", chr(code) + m[0][1])

Excerpts from output:

i = 0001
i = 0002
...
i = 0999
i = 1000 Unique_code = A000
i = 1001 Unique_code = A001
i = 1002 Unique_code = A002
...
i = 1997 Unique_code = A997
i = 1998 Unique_code = A998
i = 1999 Unique_code = A999
i = 2000 Unique_code = B000
i = 2001 Unique_code = B001
i = 2002 Unique_code = B002
i = 2003 Unique_code = B003
...
i = 9997 Unique_code = I997
i = 9998 Unique_code = I998
i = 9999 Unique_code = I999

I would construct it as a string:

sn = "{:03}{:02}{:02}{:03}{:04}".format(121, 1, 1, 1, 1)

This gives sn the value '12101010010001', zero-padding the fields to the desired width. If you want to convert the result to an integer (as opposed to leaving it as a string), just use int(sn).

The expression `range(1, 5)' creates a list of numbers from 1 to 4. So in your first test, you're asking whether the first character in your serial number is equal to that list:

"1" == [1, 2, 3, 4]

Probably not...

What you probably want to know is whether a digit is in the range (i.e. from 1 to 5, I assume, not 1 to 4).

Your other hurdle is that the first character of the serial is a string, not an integer, so you would want to take the int() of the first character. But that will raise an exception if it's not a digit. So you must first test to make sure it's a digit:

if serial_list[0].isdigit() and int(serial_list[0]) in range(1, 6):

Don't worry, if it's not a digit, Python won't even try to evaluate the part after and. This is called short-circuiting.

However, I would not recommend doing it this way. Instead, simply check to make sure it is at least "1" and no more than "5", like this:

if "1" <= serial_list <= "5":

You can do the same thing with each of your tests, varying only what you're checking.

Also, you don't need to convert the serial number to a list. serial_check is a string and accessing strings by index is perfectly acceptable.

And finally, there's this pattern going on in your code:

if thing == other:
   pass
elif thing != other:
   (do something)

First, because the conditions you are testing are logical opposites, you don't need elif thing != other -- you can just say else, which means "whatever wasn't matched by any if condition."

if thing == other:
   pass
else:
   (do something)

But if you're just going to pass when the condition is met, why not just test the opposite condition to begin with? You clearly know how to write it 'cause you were putting it in the elif. Put it right in the if instead!

if thing != other:
    (do something)

Yes, each of your if statements can easily be cut in half. In the example I gave you for checking the character range, probably the easiest way to do it is using not:

if not ("1" <= serial_list <= "5"):