How to create an alphanumeric sequence like AAAA0000 and so on
We can do this with a couple common table expressions (CTEs) that generate some simple series ...

with 

letters as
(select chr(i) as letter from generate_series(65,90) i),

digits as
(select lpad(i::text,4,'0') as digit from generate_series(0,9999) i)

select l1.letter || l2.letter || l3.letter || l4.letter || d.digit

from       letters l1
cross join letters l2
cross join letters l3
cross join letters l4
cross join digits d



our first CTE is named letters
letters generates a series of numbers from 65 to 90, which happen to be the ascii codes for letters 'A' to 'Z' so ...
we can generate our characters via the chr(i) function call
at this point letters represents the set of characters 'A' to 'Z'
our next CTE is digits 
digits generates a series of numbers from 0 to 9999, and since we're going to be dealing with characters ...
lpad(i::text,4,'0') allows us to convert this series of numbers to left/zero padded character strings
at this point digits represents a set of characters '0000', '0001', ... '9999'
from here we just need to perform a series of cross joins and append our letters/digits together


Here's a dbfiddle for the above.

For the dbfiddle you can remove the limit/offset clause to generate the complete set, alternatively adjust the limit/offset clause to see a range of outputs.

For example, with limit 20 offset 59990 we get:

?column?
--------
AAAF9990
AAAF9991
AAAF9992
AAAF9993
AAAF9994
AAAF9995
AAAF9996
AAAF9997
AAAF9998
AAAF9999
AAAG0000
AAAG0001
AAAG0002
AAAG0003
AAAG0004
AAAG0005
AAAG0006
AAAG0007
AAAG0008
AAAG0009